x86_64: minor bignum performance boost

Incorporate a few x86_64 assembly routines from the latest GMP,
which provides a small speed boost for operations such as bignum
multiplication and division.
add syntax/wrap-modbeg
scribble/manual: break `defthing` output lines if needed

If the defined identifier plus contract doesn't fit on a single
line, put them on separate lines.
scribble/manual: improve `defstruct` layout

The old layout used column spans that created ugly space around parentheses
for some combinations of field-name and keyword-modifier lengths. The new
layot avoids the problem by breaking the keyword modifiers into their
own table.