Add assertions utilities

[bitwizeshift]

Blocked by #6

The builtin assertions are lackluster for testing in that they only cover assert_eq, assert_ne, assert, and assert_match (nightly). Ideally we should have the full collection of unit test assertion helpers like mature unit test libraries have, such as GTest.

This is to track adding assertions for eq, ne, true, false, regex, match, etc.
These should make use of the test-return-types, which also enables adding simple succeed!() and fail!() macros.

[bitwizeshift]

As per #15, this might need to be revisited at a later time.

Alternatively, both assertions and non-panicking-assertions can be provided. For example, perhaps doing:

• assert_* -> treated as a panic-based assert (like assert!(...) is in the standard), and
• require_* -> treated as a non-panicking-based assert, usable in all non should_panic tests