Improvements to `Indexable` trait

[jemc]

• add small docstring (still missing docstrings for the methods)
• add required size method
• add default implementation of each_with_index and reverse_each_with_index methods based on size and []! methods
• add first! and last! methods with default implementation based on size and []! methods
• refactor tests to ensure that pure Indexable implementations are tested correctly (rather than testing the overrides implemented by Array)
• small compiler fix to ensure yield types of default implementations are traversed/resolved correctly