src/parser/parse-expr-{proto,impl}.lisp: Fully hide the parser state.

src/parser/parse-expr-{proto,impl}.lisp: Fully hide the parser state.

We didn't publish the ability to pop things, so external implementers
are rather limited in their options.  Withdraw this entirely.

The `push-operator' and `apply-operator' generic functions are now
internal.  The `push-value' function is gone entirely: the only caller
was `parse-expression', and that now simply pushes the value onto the
stack directly.