[crypto] Add bigint_copy() as a convenient wrapper macro

[crypto] Add bigint_copy() as a convenient wrapper macro

Big integers may be efficiently copied using bigint_shrink() (which
will always copy only the size of the destination integer), but this
is potentially confusing to a reader of the code.

Provide bigint_copy() as an alias for bigint_shrink() so that the
intention of the calling code may be more obvious.

Signed-off-by: Michael Brown <mcb30@ipxe.org>