Containers and commas

[KronicDeth]

Implementing commas as operators in #29 does not makes sense as unlike other operators, commas should parse a list of expressions (that don't themselves contain commas) instead of nested binary operations. elixir_parser.yrl is able to get away with parsing commas as binary expressions because the yrl format allows the elements of a given rule to be concatenated into a flattened list. To do this in the bnf format for Grammar Kit I need to actually write a list like expression, which means separating operations into a different rule than comma so that comma only contains comma-less direct children to prevent left-recursion.

[KronicDeth]

Closing in favor of smaller, decomposed issues: #66, #67, #68, and #69