* Optimize Marshal dump of large fixnum

Marshal's FIXNUM type only supports 31-bit fixnums, so on 64-bit
platforms the 63-bit fixnums need to be represented in Marshal's
BIGNUM.

Previously this was done by converting to a bugnum and serializing the
bignum object.

This commit avoids allocating the intermediate bignum object, instead
outputting the T_FIXNUM directly to a Marshal bignum. This maintains the
same representation as the previous implementation, including not using
LINKs for these large fixnums (an artifact of the previous
implementation always allocating a new BIGNUM).

This commit also avoids unnecessary st_lookups on immediate values,
which we know will not be in that table.

* Fastpath for loading FIXNUM from Marshal bignum

* Run update-deps