Try and break some dependencies. Remove unnecessary "Type" builtin

* src/opslexp.ml: Rename from typecheck.ml.  Add erasure code.
* src/lparse.ml (get_type0): Remove.
(empty_lctx): Remove.
(default_lctx, default_rctx): Merge.  Add "Type" and "Type1".
* src/lexp.ml (vdef, vref): Move to util.ml.
* src/util.ml (vdef, vref): Move from lexp.ml.
* src/env.ml: Don't open Myers.  Remove false dependency on Debruijn.
* src/elexp.ml: Don't depend on Lexp.  Move erasure code to opslexp.ml.
* src/debruijn.ml (env_extend): Inline the functions it uses.
(senv_add_var, env_add_var_info): Remove.
* src/builtin.ml (type2): New var.
(sexp_eq): Use sexp_equal.
* btl/types.typer (Type): Remove.