Expose the underlying connection from Rainbow Database so it's easier to use Dapper alongside other DB frameworks (#1257)

# Conflicts:
#	Dapper/SqlMapper.cs

* Pass LocalBuilders instead of indexes since ILGenerator already optimizes the opcode used

* Eradicate magic local indexes to make refactoring safer

resolve https://github.com/StackExchange/Dapper/pull/1313/files/ed78dd6a447b1dac20e0bba55848b6fbf87198e6#r318100560

make sure System.Data and Microsoft.Data tests can't squabble; record which Microsoft.Data tests *will never work*

drop the SqlClient ref and associated API; this makes it a (minor) breaking change, so rev the semver

detect IEnumerable<T> for some T : IDataRecord dynamically, rather than via a hard-coded handler (AddSqlDataRecordsTypeHandler)