compiletest: Run tests in their own thread to avoid blocking the scheduler

The calls to waitpid are interacting badly with the message passing that goes
on between schedulers and causing us to have very little parallelism in
the test suite. I don't fully understand the sequence of events that causes
the problem here but clearly blocking on waitpid is something that a
well-behaved task should not be doing.

Unfortunately this adds quite a bit of overhead to each test: one thread, two
tasks, three stacks, so there's a tradeoff. The time to execute run-pass on
my 4-core machine goes from ~750s to ~300s.
Cleanup: remove useless parameter
Test for issue #6338.
Pointers to bound variables are stored after all patterns are matched.

Pointers to bound variables shouldn't be stored before checking pattern,
otherwise piped patterns can conflict with each other (issue #6338).

Closes #6338.
Fix error message when trait method ends with wrong token

When parsing a trait function, the function must end with either `;` or
`{` (signifying a default implementation). The error message incorrectly
stated that it must be `;` or `}`.

Fixes #6610.
Better error message for unknown start of token

The span was fixed at some point to point to the correct character, but
the error message is still bad. Update it to emit the actual character
in question (potentially escaped).

Fixes #3747.
doc: correct spelling in documentation.
doc: convert remaining uses of core:: to std::.
Implement DeepClone for str types.
Implement Clone and DeepClone for time types.
Add UUID support to libextra
- generate random UUIDs
- convert to and from strings and bytes
- parse common string formats
- implements Zero, Clone, FromStr, ToStr, Eq, TotalEq and Rand
- unit tests and documentation
- parsing error codes and strings
- incorporate feedback from PR review
rt: Remove rust_env
Remove the annihilate lang item
rt: Remove rust_signal.h
rt: Remove unused parts of rust_globals.h
rt: Remove unused parts of rust_log
rt: Remove rust_refcount.h
rt: Remove rust_stack
rt: Remove unused rust_clone_type_desc declaration
rt: Remove empty rust_upcall.h