Add support for trait aliases

Trait aliases are currently a nightly feature.

Example code:

pub trait ValidGrouping<G> {
    type IsAggregate;
}

pub trait MixedAggregates {
    type Output;
}

mod is_aggregate {
    pub struct No;
}

pub trait NonAggregate = ValidGrouping<()>
where
    <Self as ValidGrouping<()>>::IsAggregate:
        MixedAggregates<is_aggregate::No, Output = is_aggregate::No>;

rust_analyzer seems not to be able to parse this correctly and returns a bunch of syntax errors:

383: rust-analyzer: Syntax Error: expected `{`
383: rust-analyzer: Syntax Error: expected an item
383: rust-analyzer: Syntax Error: expected EXCL
383: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
383: rust-analyzer: Syntax Error: expected SEMI
383: rust-analyzer: Syntax Error: expected an item
383: rust-analyzer: Syntax Error: expected an item
383: rust-analyzer: Syntax Error: expected an item
383: rust-analyzer: Syntax Error: expected an item
384: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected EXCL
385: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
385: rust-analyzer: Syntax Error: expected SEMI
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected EXCL
385: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
385: rust-analyzer: Syntax Error: expected SEMI
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected an item
385: rust-analyzer: Syntax Error: expected EXCL
385: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
385: rust-analyzer: Syntax Error: expected SEMI
385: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected EXCL
386: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
386: rust-analyzer: Syntax Error: expected SEMI
386: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected EXCL
386: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
386: rust-analyzer: Syntax Error: expected SEMI
386: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected EXCL
386: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
386: rust-analyzer: Syntax Error: expected SEMI
386: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected EXCL
386: rust-analyzer: Syntax Error: expected `{`, `[`, `(`
386: rust-analyzer: Syntax Error: expected SEMI
386: rust-analyzer: Syntax Error: expected an item
386: rust-analyzer: Syntax Error: expected an item