This patch fixes support for complex types.

It adds the entrypoints:
gcc_jit_context_new_rvalue_from_complex_double
gcc_jit_context_set_bool_enable_complex_types

Aswell as the binary operator GCC_JIT_BINARY_OP_COMPLEX, to make
complex numbers from two real number rvalues.

Note that the complex types already are in the type enum, so I thought the
cleanest solution is to enable them, rather than to have a "get_complex_type"
function.

When I wrote the patch originally, the complex enums were not usable due to a
range check, but I think they accidentally were included when int8_t etc were added.
However, casts to complex types segfaults currently and some other issues, so I would say
an explicit enable is the least confusing, since the enums have been there since the get go.

See attachement.