Fixes #1538 and #1545 by making DbString.ToString() much more descriptive.

Fixes #866. This provides a more informative error, saying _which_ splitOn column couldn't be found in the result set.

The used Map method is asynchronous, hence without waiting for operation to complete the benchmark is only counting the query invocation time.

Fix typo in index.md

`FirstOrDefault` is probably meant to be used here, as there is selected and returned a single record by ID.

Also changing the benchmark description, as "ExecuteReader" looks to be an obsolete name of the method that was used before refactoring.

The used Map method is asynchronous, hence without waiting for operation to complete the benchmark is only counting the query invocation time.

Add basic RepoDB benchmark

[SqlMarshal](https://github.com/kant2002/sqlmarshal) is NativeAOT friendly mini-ORM which uses source code generators.

[SqlMarshal](https://github.com/kant2002/sqlmarshal) is NativeAOT friendly mini-ORM which uses source code generators.

```
// * Summary *

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.19042
Intel Core i7-6700HQ CPU 2.60GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=5.0.201
  [Host]   : .NET Core 3.1.13 (CoreCLR 4.700.21.11102, CoreFX 4.700.21.11602), X64 RyuJIT
  ShortRun : .NET Core 3.1.13 (CoreCLR 4.700.21.11102, CoreFX 4.700.21.11602), X64 RyuJIT

|            ORM |                        Method |  Return |     Mean |    StdDev |     Error |   Gen 0 |  Gen 1 |  Gen 2 | Allocated |
|--------------- |------------------------------ |-------- |---------:|----------:|----------:|--------:|-------:|-------:|----------:|
|       Belgrade |                 ExecuteReader |    Post | 119.0 us |  22.26 us |  37.41 us |  2.6250 | 1.0000 |      - |  13.53 KB |
|     Hand Coded |                     DataTable | dynamic | 123.9 us |   2.95 us |   5.64 us |  3.0000 |      - |      - |   9.37 KB |
|     Hand Coded |                    SqlCommand |    Post | 124.8 us |  16.91 us |  32.33 us |  1.2500 | 0.5000 |      - |   7.42 KB |
|     SqlMarshal |                    SqlCommand |    Post | 132.8 us |   2.77 us |   4.66 us |  2.0000 | 0.7500 |      - |  11.27 KB |
|         Dapper |  QueryFirstOrDefault<dynamic> | dynamic | 135.0 us |   3.64 us |   6.96 us |  3.5000 |      - |      - |  11.39 KB |
|     LINQ to DB |                      Query<T> |    Post | 135.9 us |   3.52 us |   5.92 us |  1.7500 | 0.7500 |      - |  10.64 KB |
|     LINQ to DB |            'First (Compiled)' |    Post | 136.5 us |   2.99 us |   5.72 us |  1.7500 | 0.7500 |      - |  11.23 KB |
|       PetaPoco |             'Fetch<T> (Fast)' |    Post | 136.8 us |   4.67 us |   7.85 us |  2.0000 | 1.0000 |      - |  11.52 KB |
|         Dapper |   'Query<dynamic> (buffered)' | dynamic | 137.6 us |   6.44 us |  12.31 us |  2.0000 | 1.0000 |      - |  11.72 KB |
|         Mighty |                Query<dynamic> | dynamic | 141.7 us |   4.99 us |   9.54 us |  2.0000 | 1.0000 |      - |  12.43 KB |
|        Massive |             'Query (dynamic)' | dynamic | 142.1 us |   1.59 us |   2.66 us |  2.0000 | 1.0000 |      - |  12.07 KB |
|         Dapper |         'Query<T> (buffered)' |    Post | 143.0 us |   7.98 us |  13.41 us |  2.0000 | 1.0000 |      - |  11.64 KB |
|         Mighty |      SingleFromQuery<dynamic> | dynamic | 144.8 us |   3.95 us |   6.64 us |  2.5000 | 1.0000 |      - |  12.43 KB |
|         Dapper |        QueryFirstOrDefault<T> |    Post | 147.0 us |  22.39 us |  42.80 us |  1.7500 | 0.7500 |      - |  11.35 KB |
|       PetaPoco |                      Fetch<T> |    Post | 149.2 us |  22.91 us |  43.80 us |  2.5000 | 1.0000 | 0.2500 |   12.2 KB |
|         Dapper |              'Contrib Get<T>' |    Post | 149.9 us |  13.30 us |  25.42 us |  2.0000 | 1.0000 |      - |  12.28 KB |
|   ServiceStack |                 SingleById<T> |    Post | 151.8 us |   3.95 us |   7.56 us |  2.7500 | 1.0000 | 0.2500 |  15.27 KB |
|         Mighty |            SingleFromQuery<T> |    Post | 152.5 us |  14.77 us |  24.83 us |  2.0000 | 1.0000 |      - |  12.53 KB |
|         Mighty |                      Query<T> |    Post | 162.7 us |  45.49 us |  76.44 us |  2.0000 | 1.0000 |      - |  12.53 KB |
|         Dapper |       'Query<T> (unbuffered)' |    Post | 191.4 us |   6.95 us |  11.67 us |  2.2500 | 1.0000 | 0.2500 |  11.76 KB |
|     LINQ to DB |                         First |    Post | 200.6 us |  24.67 us |  47.18 us |  2.7500 | 1.0000 | 0.2500 |  15.79 KB |
| DevExpress.XPO |             GetObjectByKey<T> |    Post | 204.8 us |   5.69 us |  10.89 us |  5.5000 | 1.5000 |      - |  29.16 KB |
|        EF Core |            'First (Compiled)' |    Post | 212.4 us |  15.70 us |  30.01 us |  2.5000 |      - |      - |   8.39 KB |
|           EF 6 |                      SqlQuery |    Post | 251.1 us |  92.97 us | 140.55 us |  3.7500 | 1.0000 |      - |  23.67 KB |
|         Dapper | 'Query<dynamic> (unbuffered)' | dynamic | 255.9 us |  99.08 us | 149.80 us |  2.5000 | 1.0000 | 0.2500 |   11.8 KB |
| DevExpress.XPO |                      Query<T> |    Post | 285.4 us |  92.67 us | 140.10 us | 10.2500 |      - |      - |  31.71 KB |
|     NHibernate |                           HQL |    Post | 289.8 us |  18.26 us |  34.92 us |  5.5000 | 1.0000 |      - |  31.32 KB |
|     NHibernate |                        Get<T> |    Post | 302.1 us |  16.69 us |  28.05 us |  5.5000 | 1.0000 |      - |  29.37 KB |
|        EF Core |         'First (No Tracking)' |    Post | 334.9 us |  50.05 us |  84.11 us |  3.5000 | 0.5000 |      - |  19.53 KB |
|           EF 6 |                         First |    Post | 339.4 us |  43.67 us |  66.03 us | 14.0000 |      - |      - |  44.18 KB |
|        EF Core |                      SqlQuery |    Post | 344.0 us |  55.47 us |  83.86 us |  5.5000 |      - |      - |  17.59 KB |
|        EF Core |                         First |    Post | 357.5 us |  51.72 us |  78.20 us |  4.0000 |      - |      - |   12.3 KB |
|     NHibernate |                      Criteria |    Post | 357.6 us |  37.63 us |  63.23 us | 11.5000 | 1.0000 |      - |   56.5 KB |
|           EF 6 |         'First (No Tracking)' |    Post | 383.1 us |  33.98 us |  57.10 us |  9.0000 | 1.0000 |      - |  50.95 KB |
|     NHibernate |                           SQL |    Post | 399.2 us |  40.65 us |  68.31 us | 19.0000 | 1.0000 |      - |   78.9 KB |
| DevExpress.XPO |                 FindObject<T> |    Post | 602.5 us | 361.92 us | 547.18 us |  8.0000 |      - |      - |   27.2 KB |
|     NHibernate |                          LINQ |    Post | 925.9 us |  47.62 us |  80.02 us | 12.0000 | 2.0000 |      - |  53.59 KB |
```

And thus can be removed

Fix typos