improve allocations in expression generator

[pquerna]

• We noticed heavy allocation generation in a production workflow, which then generates lots of garbage collection pressure, coming from the dataset -> toSQL generation.

• Adds benchmarks around the Expression SQL generator

• Adds pooling of instances

• Adds GrowBuffer/GrowArgs to upsize buffers when we know large writes are coming

• Adds more native / non-reflection based writers

• other minor optimizations

benchmark                                                                       old allocs     new allocs     delta
BenchmarkExpressionSQLGenerator_Insert/Unprepared/SingleRowManyCols-8           1626           1596           -1.85%
BenchmarkExpressionSQLGenerator_Insert/Unprepared/MultiRowBulk-8                35730          34695          -2.90%
BenchmarkExpressionSQLGenerator_Insert/Prepared/SingleRowManyCols-8             868            864            -0.46%
BenchmarkExpressionSQLGenerator_Insert/Prepared/MultiRowBulk-8                  11263          11239          -0.21%
BenchmarkExpressionSQLGenerator_Insert/PreparedWithNums/SingleRowManyCols-8     1025           864            -15.71%
BenchmarkExpressionSQLGenerator_Insert/PreparedWithNums/MultiRowBulk-8          19357          11239          -41.94%
BenchmarkExpressionSQLGenerator_Update/Unprepared-8                             1754           976            -44.36%
BenchmarkExpressionSQLGenerator_Update/Prepared-8                               1595           1340           -15.99%
BenchmarkExpressionSQLGenerator_Update/PreparedWithNums-8                       2240           1340           -40.18%
BenchmarkExpressionSQLGenerator_Select/Unprepared-8                             3961           2520           -36.38%
BenchmarkExpressionSQLGenerator_Select/Prepared-8                               3585           3330           -7.11%
BenchmarkExpressionSQLGenerator_Select/PreparedWithNums-8                       4912           3331           -32.19%