We use spin locks so as to not require Pthreads.

The stdlib of Ocaml already defines a `Result` module. The `Result`
module of E-ACSL was masking this module and would cause problems in
some cases.

The result type has been moved to `Error.result` and is now implemented
with the `Result` module of Ocaml.

The file `signalled.h` is now common to `format` and `builtin` tests.

- `tapp_to_exp` is moved to `Logic_functions`, with the existing
`Logic_function.tapp_to_exp` renamed to `function_to_exp` and moved as
an internal function;
- `name_of_mpz_arith_bop` is moved to `Gmp`;
- `add_cast` and `strnum` are moved to a new `Typed_number` module.