Adds loop expression

[baszalmstra]

Enabling a much faster fibonacci:

fn fibonacci(n:int):int {
    let a = 0;
    let b = 1;
    let i = 1;
    loop {
        if i > n {
            return a
        }
        let sum = a + b;
        a = b;
        b = sum;
        i += 1;
    }
}

Depends on #49

[codecov]

Codecov Report

Merging #60 into master will decrease coverage by 70.85%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #60       +/-   ##
=========================================
- Coverage   70.85%     0%   -70.86%     
=========================================
  Files         106     74       -32     
  Lines        6838   4869     -1969     
=========================================
- Hits         4845      0     -4845     
- Misses       1993   4869     +2876

Impacted Files	Coverage Δ
crates/mun_syntax/src/tests/lexer.rs	0% <ø> (-100%)	⬇️
crates/mun_hir/src/ty/infer.rs	0% <0%> (-74.66%)	⬇️
...ates/mun_syntax/src/parsing/grammar/expressions.rs	0% <0%> (-94.48%)	⬇️
crates/mun_syntax/src/ast/generated.rs	0% <0%> (-55.75%)	⬇️
crates/mun_syntax/src/ast/traits.rs	0% <0%> (-46.16%)	⬇️
crates/mun_syntax/src/syntax_kind/generated.rs	0% <0%> (-78.89%)	⬇️
crates/mun_syntax/src/tests/parser.rs	0% <0%> (-97.92%)	⬇️
crates/mun_hir/src/expr.rs	0% <0%> (-75.43%)	⬇️
crates/mun_hir/src/ty/tests.rs	0% <0%> (-95.66%)	⬇️
crates/mun_hir/src/db.rs	0% <0%> (-100%)	⬇️
... and 95 more

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[baszalmstra]

Loop statements