Allow parse_procs to be bound by subclasses.
Allow parse_procs to be bound by subclasses.

The `BindAST` template requires that AST classes have default constructors
and a `construct` method for post-construction initialization. This is fine
for some use cases, but it precludes effective patterns like
immutable fields and factory methods.

The precise details of how a language's AST nodes are constructed are
highly variable, so rather than adding more variants of `BindAST` to
Pegmatite itself, this commit makes `ASTParserDelegate::set_parse_proc`
and `bind_parse_proc` visible to subclasses of `ASTParserDelegate`.
That way, a language's parser delegate can implement its own binding logic.

charlesturner7c5/Pegmatite