Use unordered_map instead of vector for primitives

[MaartenS11]

Technically this should be faster than a linear search + it allows overwriting primitives that you already have. For example:

wac->interpreter->register_primitive("chip_digital_write", [](Module *m) -> bool {
    pop_args(2);
    printf("custom chip_digital_write behaviour!\n");
    return true;
}, &twoToNoneU32);

This overwriting is particularly useful for the concolic version of the interpreter so that it can have custom implementations that for example push symbolic variables.