parse: parser for the Hare grammar

hare::parse provides a parser for Hare source code. The [[subunit]] function
will parse a single Hare source file and return a [[hare::ast::subunit]]. Other
functions provide parsers for various important Hare sub-terminals, such as
[[decls]] and [[imports]]. See the Hare specification for more details:

https://harelang.org/specification

Most of these functions require the caller to provide a Hare lexer, see
[[hare::lex::]] for details.
