[dev.boringcrypto] crypto/internal/boring: avoid allocation in big.Int conversion

[dev.boringcrypto] crypto/internal/boring: avoid allocation in big.Int conversion

The conversion via byte slices is inefficient; we can convert via word slices
and avoid the copy entirely.

For #51940.

Change-Id: I06f747e0acffffae427d9706d43bdacf146c027d
Reviewed-on: https://go-review.googlesource.com/c/go/+/395875
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>