rustc: Refactor metadata.Encode.* to not require a type abbreviation table if abbreviation isn't enabled

duplication, but we will hopefully drop the rustboot one soon.

This is also a preparation for changing the rustc one to have the activate glue
return to the exit glue which will then call the main function.

This (returning to the function that calls main) matches what happens when
loader stats a program or a new thread. It lets gdb produce good backtraces
and should help with EH too.

This reverts commit 480eda0f. The commit
adding the assert keyword has been restored.

This reverts commit d08b443f.

This reverts commit aa25f22f. It broke stage2, not sure why yet.

While those unexported items can't be used by name outside the module in which
they are defined, they can be used as a sort of ADT.

This giant commit changes the syntax of Rust to use "assert" for
"check" expressions that didn't mean anything to the typestate
system, and continue using "check" for checks that are used as
part of typestate checking.

Most of the changes are just replacing "check" with "assert" in test
cases and rustc.

It's not actually involved in the fold so it can just be passed between the
functions that need it.

An unexported foo.baz can't be resolved from inside foo when called as foo.baz
instead of just baz. This behavior may want to change eventually.

This avoids outputing, e.g. /test.bc for ../test.rs

doc fix