Update compile-time option documentation.
Remove `MLton.Socket` structure (deprecated in last release).
Remove 'MLton.Socket' structure from stubs.
Remove unused CM files.
Update 'MLTON_PLATFORM' signature documentation.
Updates to Basis Library implementation for 'resolveScope' annotation.

This commit ensures that the Basis Library implementation type checks
with all 'resolveScope' annotations.  Only compilation with
'resolveScope dec' required changes.

One disappointing aspect of 'resolveScope dec' is that it can force
the resolution of flexible record patterns at an "inner" declaration,
rather than an "outer" declaration.  That is, 'resolveScope dec'
forces resolution at the conclusion of each elaborated declaration,
and forces the resolution of all flexible record added since the last
elaborated declaration.