crypto/ecdh,crypto/internal/nistec: enable pruning of unused curves

crypto/ecdh,crypto/internal/nistec: enable pruning of unused curves

If a program only uses ecdh.P256(), the implementation of the other
curves shouldn't end up in the binary. This mostly required moving some
operations from init() time. Small performance hit in uncompressed
Bytes/SetBytes, but not big enough to show up in higher-level
benchmarks. If it becomes a problem, we can fix it by pregenerating the
p-1 bytes representation in generate.go.

For #52182
Updates #52221

Change-Id: I64460973b59ee3df787d7e967a6c2bcbc114ba65
Reviewed-on: https://go-review.googlesource.com/c/go/+/402555
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Fernando Lobato Meeser <felobato@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>