Support "noalloc" and "float" for generated stubs

[yallop]

Adding the "noalloc" and "float" attributes to generated external bindings where appropriate would improve performance.

The type-based analyses that determine whether a function

• may allocate or
• operates directly on floats

are implemented, but have no way of determining whether a C function calls back into the OCaml runtime, violating the invariants. We could add an additional flag to foreign that serves as a promise that the bound function doesn't call back OCaml.