(#7523) Refactor the grammar to reduce duplication

This commit unifies some paths in the grammar, which had previously been
duplicated to avoid shift/reduce conflicts. Merging these paths together and
separating only the conflicting structures leads to a cleaner grammar, with
fewer holes.

Several bugs are fixed as a result:

(#3129) Nested class names beginning with numbers work correctly
(#5268) Hyphens in class names work correctly
(#5817) Hashes and arrays can now be passed to functions (hashes require parentheses)

Additionally, expressions are now legal in most places where they would make
sense, when previously only bare rvalues were allowed.

Paired-With: Markus Roberts
Reviewed-By: Matt Robinson