(For now only fail when parse_from_source_str is used to avoid
possible compatibility problems; parse_expr_from_source_str still
does not check.)

Note: part from Niko Matsakis commit: rewrite assert to accept a $ in
0th pos.

Fixes issue #1785.

for better error messages.

that are really a substr of another file.

Closes #1586

Closes #1475

They are now a (dictptr, box) pair, where box is a regular boxed
representation of the inner type. This cuts down on some special
case code.

Also removes some code from trans::base that was moved to shape but
then somehow restored in trans::base through a bad merge.

Starts on issue #1567

Closes #1803

Closes #1802

Joinable pthreads need to be either joined or detached and we no
longer join with the scheduler threads.

Changes to resolve and typeck. Still nothning working yet.