This adds float type support to the main switch blocks in Read and
Write, instead of falling back to reflection. This gives a considerable
speedup for the float types:

ReadFloats-8                 129ns ± 9%       70ns ± 8%   -46.02%  (p=0.001 n=7+7)
WriteFloats-8                131ns ± 6%       86ns ±11%   -34.59%  (p=0.001 n=7+7)
ReadSlice1000Float32s-8     14.6µs ±14%      4.8µs ±12%   -67.29%  (p=0.001 n=7+7)
WriteSlice1000Float32s-8    16.4µs ±20%      4.7µs ± 8%   -71.01%  (p=0.001 n=7+7)

Change-Id: I0be99d068b07d10dd6eb1137b45eff6f7c216b87
GitHub-Last-Rev: 4ff326e99ca35977d819f0ba29c10d9efc7e811c
GitHub-Pull-Request: golang/go#31803
Reviewed-on: https://go-review.googlesource.com/c/go/+/174959Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>