Added optional type system - complain if proven wrong

[bvssvni]

See #80

This PR adds an optional type system using a similar, but different syntax than Rust.

• bool (boolean)
• f64 (number)
• vec4 (4D vector)
• []/[T] (array)
• {} (object)
• opt[T] (option)
• res[T] (result)
• any (any type, except void)
• void (no value type)

Example

fn foo() -> opt[bool] {
    return some(true)
}

fn main() {
    if foo() {
        println("oh?")
    }
}

 --- ERROR --- 
In `source/test.rs`:
Type mismatch: Expected `bool`, found `opt[bool]`
6,8:     if foo() {
6,8:        ^

Any

An any type works with any other type except void.

Non-guarantee of argument type inside function body

Consider a simple example like this:

fn foo(a: bool) { ... }

It is not guaranteed that a has the type bool inside the function body. This is because the function can be called with an argument that is inferred to any at type checking.

Performance

The runtime performance should not be affected significantly, but the loading will take a little extra time.

Before:
test tests::bench_add         ... bench:  43,908,267 ns/iter (+/- 5,914,741)
test tests::bench_add_n       ... bench:  15,829,695 ns/iter (+/- 2,857,750)
test tests::bench_array       ... bench:  18,964,583 ns/iter (+/- 2,814,946)
test tests::bench_call        ... bench:  17,429,751 ns/iter (+/- 3,573,531)
test tests::bench_len         ... bench:   3,572,701 ns/iter (+/- 207,784)
test tests::bench_main        ... bench:   2,015,865 ns/iter (+/- 93,179) <---
test tests::bench_min         ... bench:  17,232,992 ns/iter (+/- 2,241,279)
test tests::bench_min_fn      ... bench:  81,652,195 ns/iter (+/- 5,507,277)
test tests::bench_n_body      ... bench:  76,455,628 ns/iter (+/- 4,989,369)
test tests::bench_object      ... bench:  26,533,172 ns/iter (+/- 3,704,094)
test tests::bench_primes      ... bench:  48,117,161 ns/iter (+/- 5,277,033)
test tests::bench_primes_trad ... bench:  47,062,226 ns/iter (+/- 7,232,953)
test tests::bench_sum         ... bench:   7,063,747 ns/iter (+/- 1,363,337)

After:
test tests::bench_add         ... bench:  44,384,373 ns/iter (+/- 5,417,288)
test tests::bench_add_n       ... bench:  17,259,780 ns/iter (+/- 2,925,107)
test tests::bench_array       ... bench:  19,701,937 ns/iter (+/- 3,805,578)
test tests::bench_call        ... bench:  18,241,928 ns/iter (+/- 3,989,516)
test tests::bench_len         ... bench:   3,745,375 ns/iter (+/- 240,261)
test tests::bench_main        ... bench:   2,196,033 ns/iter (+/- 114,887) <---
test tests::bench_min         ... bench:  17,259,160 ns/iter (+/- 2,536,126)
test tests::bench_min_fn      ... bench:  82,869,271 ns/iter (+/- 7,882,435)
test tests::bench_n_body      ... bench:  76,156,329 ns/iter (+/- 6,425,743)
test tests::bench_object      ... bench:  26,784,073 ns/iter (+/- 4,257,977)
test tests::bench_primes      ... bench:  48,005,609 ns/iter (+/- 5,273,527)
test tests::bench_primes_trad ... bench:  46,856,265 ns/iter (+/- 5,475,704)
test tests::bench_sum         ... bench:   7,120,835 ns/iter (+/- 654,078)

[bvssvni]

This is just very basic type checking, but merging because it seem to work reasonably well and the more advanced stuff needs tweaking over longer time.