Add runtime asserts that validates the provided enums

[emgre]

If a C library calls a Rust function with an unknown enum value, the behaviour is undefined. The Rust code generator should probably take enums as integer than check if the value fits with one of the enum variant. If it doesn't fit, it would pass None to our code.

Resources:

• https://users.rust-lang.org/t/guarding-against-invalid-enum-values-arriving-via-ffi/33482
• Match on repr(C) enum returned from C library with unknown value leads to UB rust-lang/rust#36927

[emgre]

Instead of an option, maybe only add a runtime assert that panics instead.

[emgre]

Fixed in #28.