Add assertions utilities

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.

Start by reviewing the blocker in issue #6 and the test-return-types work referenced here. Compare the requested eq, ne, true, false, regex, and match assertions with the existing built-in assertions; done means the assertion collection and the simple succeed!() and fail!() macros are supported.

Written by the indexing model from the issue text.