Change root binding of Vars

[jfacorro]

There are some functions/macros that rely on being able to change the root binding of Vars (e.g. with-redefs-fn). In Clojerl the root binding for a Var is returned by a function with a __val suffix, the compiler makes sure the root binding is always a literal. When the Var is a function the root binding is the Var itself, and also a function with the same name is generated (e.g. #'clojure.core/inc is backed by the functions inc__val/0, which returns #'clojure.core/inc itelf, and inc/1).

Alternatives:

1. Redefine with def : would mean the whole module is re-compiled, which is somewhat expensive in some cases.
2. Store root bindings in ETS: introducing another indirection will make accessing the root binding slower for all Vars.
3. Dynamic binding approach: it wouldn't actually be changing the root binding, but the value returned by non-dynamic Vars could be changed on a process basis.
4. Don't support changing root binding of vars.

Related tests:

• clojure.test-clojure.vars