Improve stdlib performance with large result sets
Improve stdlib performance with large result sets

In a hot path type assertions are expensive. Cache the already correctly
typed interfaces. ~20% improvement with 1000 rows.

Before:

jack@glados ~/dev/pgx/stdlib ±master » PGX_BENCH_SELECT_ROWS_COUNTS='1 10 1000' got ./... -bench=SelectRows -benchmem
goos: darwin
goarch: amd64
pkg: github.com/jackc/pgx/v4/stdlib
BenchmarkSelectRowsScanSimple/1_rows-16         	   21465	     55060 ns/op	    1679 B/op	      40 allocs/op
BenchmarkSelectRowsScanSimple/10_rows-16        	   16692	     71176 ns/op	    3827 B/op	     148 allocs/op
BenchmarkSelectRowsScanSimple/1000_rows-16      	     800	   1369547 ns/op	  248855 B/op	   12938 allocs/op
BenchmarkSelectRowsScanNull/1_rows-16           	   20306	     57883 ns/op	    1940 B/op	      54 allocs/op
BenchmarkSelectRowsScanNull/10_rows-16          	   15942	     74729 ns/op	    4294 B/op	     171 allocs/op
BenchmarkSelectRowsScanNull/1000_rows-16        	     829	   1326788 ns/op	  261291 B/op	   13051 allocs/op
PASS
ok  	github.com/jackc/pgx/v4/stdlib	10.429s

After:

jack@glados ~/dev/pgx/stdlib ±master⚡ » PGX_BENCH_SELECT_ROWS_COUNTS='1 10 1000' got ./... -bench=SelectRows -benchmem
goos: darwin
goarch: amd64
pkg: github.com/jackc/pgx/v4/stdlib
BenchmarkSelectRowsScanSimple/1_rows-16         	   21327	     55097 ns/op	    2127 B/op	      43 allocs/op
BenchmarkSelectRowsScanSimple/10_rows-16        	   16724	     69496 ns/op	    4276 B/op	     151 allocs/op
BenchmarkSelectRowsScanSimple/1000_rows-16      	    1009	   1124573 ns/op	  250037 B/op	   12941 allocs/op
BenchmarkSelectRowsScanNull/1_rows-16           	   20577	     58117 ns/op	    2396 B/op	      57 allocs/op
BenchmarkSelectRowsScanNull/10_rows-16          	   16402	     72533 ns/op	    4750 B/op	     174 allocs/op
BenchmarkSelectRowsScanNull/1000_rows-16        	    1010	   1161437 ns/op	  261735 B/op	   13054 allocs/op
PASS
ok  	github.com/jackc/pgx/v4/stdlib	10.363s

jackc/pgx,jackc/pgx,jackc/pgx,jackc/pgx