Add preliminary support for big-ints

* GNUmakefile (OBFLAGS): Add `nums`.
* src/debruijn.ml (type_integer): New type.
* src/builtin.ml (register_builtin_csts): Register it.
* src/env.ml (value_type): Add big-int constructor.
* src/eval.ml (add_binary_biop, add_binary_bool_biop): New functions.
Use them to setup new primitives.
* btl/builtins.typer (Integer_*): Give them types.