Further formatting improvements

[Alasdair]

This file makes two main improvements to the formatter. First and most importantly, it adds an AST diff check formatting. This means that if the syntax tree differs in any way (other than some minor permitted differences) after reformatting, this raises an error.

Secondly, we now preserve lined up match statements where possible, so in something like:

match x {
  0  => foo,
  32 => bar,
  _  => baz,
} 

The extra spaces after 0 and _ will be preserved, allowing the => to remain lined up. This happens when we detect that all the match expressions were lined up in the original source. This heuristic likely needs some tweaking to find the correct balance.

[github-actions]

Test Results

   16 files     36 suites   0s ⏱️
1 004 tests 1 001 ✅  3 💤 0 ❌
4 840 runs  4 798 ✅ 42 💤 0 ❌

Results for commit 16dbbe9.