Add non-fatal assertions

[bitwizeshift]

As an extension to #7, it can be useful in testing to provide non-fatal assertions for tests, which can allow the code to progress after failure.

The current proposed implementation is:

• Extend __Context to support storing assertion errors
• Add a proc-macro that writes to the implicit __context for failures (this is necessary since declarative macros will fail due to __context not being visible in the current scope)
• Create declarative macros that call the proc-macro to trigger errors

There should also be considerable code-sharing between #7 and this, since anything we want asserts of, we should also offer expects.