A few cleanups and minor improvements to rustc/traits

[ljedrz]

It's a little bigger than usual, so bear with me ^^:

• introduce TyCtxt::all_impls and use it to avoid inefficiently allocating push loops
• modify ArgKind::from_expected_ty to take an Option<Span> argument to make it more versatile
• replace ArgKind::Arg("_".to_owned(), "_".to_owned()) with ArgKind::empty
• move early returns earlier where possible
• if all branches of a match end with the same expression, move it after it
• change a hacky match expression to an if else chain
• move the push out from a push loop closure to reduce the number of allocations
• correct the vector size for pretty_predicates (under specialize)
• take advantage of the fact that if else is an expression
• prefer cloned() to map(|&x| x) and map(|x| *x)
• prefer vec![x; y.len()] to y.map(|_| x).collect()
• use unwrap_or_else instead of match where applicable
• use if let instead of match when only one branch matters
• prefer to_owned to to_string for string literals
• remove explicit returns
• remove superfluous braces
• whitespace fixes
• several other minor improvements

[rust-highfive]

r? @eddyb

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[ljedrz]

I just noticed that I forgot to take advantage of the predicate_to_obligation (previously push) closure in select; that's the only update.

[eddyb]

r? @nikomatsakis

[nikomatsakis]

@bors r+ rollup

[bors]

📌 Commit a8ec8e5 has been approved by nikomatsakis