fix the general -> primary -> parenthesized expression hierarchy

[janw4ld]

this PR

• renames _prefix_expression to _primary_expression to match a more common & conventional name
• correctly categorises all primary expression kinds in the grammar as such
• makes _primary_expression a child/subkind of _expression
• makes parenthesized_expression a child of _primary_expression

this fixes a class of shortcomings that prevent parsing parenthesised expressions that evaluate to simple objects from being treated as objects everywhere in the grammar.

this addresses the first shortcoming stated in #28

[murtaza64]

It seems like you know the Groovy grammar better than I do. Out of curiosity, was this stuff described in the language spec at all?

[janw4ld]

not really, i haven't read the language spec but im learning about programming language design in general and comparing groovy to java and python