Parse expressions with precedence and associativity

Currently, the parser requires every function application to be parenthesized. Lifting this restriction would require making the parser aware of precedence and associativity rules.

Ideally, #3 should be implemented first.

Start by reviewing the parser and issue #3, which should be implemented first. Define how unparenthesized function applications interact with precedence and associativity, and consider the work complete when expressions no longer require every function application to be parenthesized.

Written by the indexing model from the issue text.