Use the existing pass through reflect to find field by name

This greatly sped up reflect tests somehow.

benchmark                               old ns/op     new ns/op     delta
BenchmarkReflectRoot                    4.66          5.35          +14.81%
BenchmarkReflectToplevelExact           2235          758           -66.09%
BenchmarkReflectToplevelTagged          2336          671           -71.28%
BenchmarkReflectToplevelTaggedLower     568           638           +12.32%
BenchmarkReflectDeep                    2002          1645          -17.83%
BenchmarkReflectSlash                   1939          1811          -6.60%
BenchmarkReflectTilde                   1904          1668          -12.39%

benchmark                               old allocs     new allocs     delta
BenchmarkReflectRoot                    0              0              +0.00%
BenchmarkReflectToplevelExact           8              3              -62.50%
BenchmarkReflectToplevelTagged          8              2              -75.00%
BenchmarkReflectToplevelTaggedLower     2              2              +0.00%
BenchmarkReflectDeep                    6              6              +0.00%
BenchmarkReflectSlash                   7              7              +0.00%
BenchmarkReflectTilde                   6              6              +0.00%

benchmark                               old bytes     new bytes     delta
BenchmarkReflectRoot                    0             0             +0.00%
BenchmarkReflectToplevelExact           136           51            -62.50%
BenchmarkReflectToplevelTagged          136           42            -69.12%
BenchmarkReflectToplevelTaggedLower     42            42            +0.00%
BenchmarkReflectDeep                    159           151           -5.03%
BenchmarkReflectSlash                   119           119           +0.00%
BenchmarkReflectTilde                   111           111           +0.00%