Some work on type mapping to make everything work.
Every mapping now has to be one-to-one: a mapping returned for a CLR
type must contain that same type; previously when looking up by
DateTimeOffset we would return a timestamptz mapping which specified
CLR DateTime.

We now have separate mappings for DateTimeOffset and DateTime
(as well as for byte and short), and can handle multiple mappings
given a store type.

Fixes #347

Which was double-prefixed. This unfortunately breaks any existing
migrations which specify index methods.

Fixes #346

Closes #342

Closes #339

Still some work to do

* Store type lookup is now case-insensitive
* Fixed NpgsqlTimeTypeMapping to map to TimeSpan, not DateTime

Closes #332

Thanks @lust4life

Closes #327

Scaffolding types with precision/scale works now

Fixes #319

Closes #270

Fixes #286

Fixes #288

This reverts commit dfdc3364.

Pending resolution of
https://github.com/aspnet/EntityFrameworkCore/issues/11077

* Added missing mapping DateTimeOffset -> timestamptz
* Adding literal generation for DateTime

* Multidimensional arrays not yet supported (#314)
* In some cases boxing will occur when comparing (and snapshotting) the
  elements (https://github.com/aspnet/EntityFrameworkCore/issues/11072)

Closes #305

Fixes #313

Closes #306