crypto/elliptic: refactor package structure

crypto/elliptic: refactor package structure

Not quite golang.org/wiki/TargetSpecific compliant, but almost.

The only substantial code change is in randFieldElement: it used to use
Params().BitSize instead of Params().N.BitLen(), which is semantically
incorrect, even if the two values are the same for all named curves.

For #52182

Change-Id: Ibc47450552afe23ea74fcf55d1d799d5d7e5487c
Reviewed-on: https://go-review.googlesource.com/c/go/+/315273
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>