gzip: code shrink

gzip: code shrink

Converted a few 16-bit variables and small arrays to 32-bit.

Stopped pulling desc->FOO members into temporary local variables
in gen_bitlen(): on register-starved arches, this is a loss,
temporaries go into stack slots.

Sprinkled a few "const" on pointer arguments.

function                                             old     new   delta
pack_gzip                                            742     745      +3
gen_codes                                            101      97      -4
build_tree                                           886     833     -53
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 3/-57)             Total: -54 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>