Minor cleanup in subtype.rkt
Cleanup requires in tc-apply.
Cleanup tc-app-helper.rkt.
racket/gui gtk: preserve button font on label change
pkg/gui: fixes for label widths
libffi fix

Closes PR 13707
pkg/gui: find an available checkmark glyph
Switch out? to from-typed? as it has the same meaning.
Allow requiring functions with filters in contravariant positions.
Make type->contract correctly respect variance.

Track whether we are protecting values from the typed side and/or the
untyped side.

Closes PR 13662.
Closes PR 13663.
Closes PR 13665.
Make scoped type vars work only for explict Alls and annotations.

Solves major issue disscussed on PR 13586.
Closes PR 13622.
Solves issue when typechecking lambdas with types like (All (a) Any).
Closes PR 13596.
Closes PR 13539.
Closes PR 13703.