vm: fix bignum_to_fixnum_strict for BIGNUM_ZERO.
compiler.tree.modular-arithmetic: strict versions shouldn't be here.
Revert "vm: fix bignum_to_fixnum_strict for BIGNUM_ZERO."

This reverts commit 0a66016c7db3c8c02af609d7c46411dc05a7b0dd.
llvm.engine: fix using.
stack-checker.known-words: adding bignum>fixnum-strict.
alien.libraries.finder: fix docs.
help.lint.checks: allow $slot to be used with struct-class.
opengl.capabilities: fix docs.
compiler.cfg: fix some docs.