Specify function selector string format and create a test suite for it

[digorithm]

As discussed FuelLabs/fuels-rs#17, we need to make sure every artifact that implements some sort of function selector string builder will follow the same specs and test their implementation against the same test suite.

For instance, a method called do_something that takes a custom type:

struct InputStruct {
    field_1: bool,
    field_2: u8
}

Should have the following function selector represented as string: do_something(s(bool,u8)). Which will then be 256-hashed and its first 4 bytes will be the final function selector.

We need a test suite/test vector with a decent coverage for this, so that whoever is implementing the same functionality can test against it.