* if we stop supporting DbDataReader, someone is going to complain; I'd
rather wrap it for now (to make the lib work), and add an analyzer
in AOT that shouts at them for using IDbConnection

also: compiler nits

* release notes

add support for fetching values via GetFieldValue<T>; this requires
DbDataReader, but in reality the reader is *always* DbDataReader;
acknowledge this, and stop pretending to use IDataReader internally
(no breaks to public API)

* initial net5.0 tweaks; also fix current IDE warnings

* lib updates and fixes

* more cleanup

* fix snafu

* skip locals init

* disable xunit shadow copy in tests

* test records

* explore scenario that currently works re #1571

This is a branch cleaning up old `#if` clauses, `.csproj` bits, Linux build compatibility, and a few other tid-bits (broken out by commit).

In general: we're on 3.0 tooling and all previous cruft has been removed.

* prep work for adding Microsoft.Data.SqlClient support/tests; SqlClient dependency is not yet removed

Related issue: #520

Update to rc1 (23516 dependencies); implement test skipping (including case-sensntivity check); swap all explicit #if to vague #if so we can iterate without bulk changes

- Expression bodies
- Auto-properties (also get-only properties)
- Null propagation
- `nameof` operator
- String interpolation
- Dictionary initializer
- await in finally clause

The "SqlMapper.cs is too damned big" file split; also removes C#3 support and .NET 3.5 support (sorry, but legacy by too many years)