* Add test_matrix macro

Allows generating Cartesian product test matrices.
Tests and docs forthcoming.

* Add test_matrix macro acceptance tests

Test basic features and for correct compilation error
output

* Add `=>` expression support to test_matrix macro

Required that `TestCaseExpression` and all it's descendant structs be
`Clone` so that TestMatrix can clone them for each TestCase it creates.
Also made TestComment Clone for parity.

Implemented quote::ToTokens for TestComment so that the "illegal
comment" error generated by TestMatrix can point to the correct span of
where the illegal comment begins. On stable Rust, this just highlights
the first token, the semicolon, but on nightly Rust (which has
`quote::Span::join`) it will highlight the entire illegal comment.

* Use isize in test_matrix ranges

`isize` will allow for negative numbers and be compatible
with 32-bit and 64-bit platforms.

Add some acceptance tests for the same.

* Add basic documentation for test_matrix macro

Add unreleased CHANGELOG fragment, README documentation,
and doc comment to src/lib.rs and crate/test-case-macros/src/lib.rs.

Detailed Wiki documentation forthcoming.

* Replace itertools dependency with copied module

Copied multi_product module from itertools 0.11.0
under MIT license. Eliminates dependency on building
all of itertools.