93: Support leading pipe in match arms r=matklad a=DJMcNab
This adds support for match arms of the form:
```rust
<...>
| X | Y => <...>,
| X => <...>,
| 1..2 => <...>,
etc
```
# Implementation discussion
This just naïvely 'eats' a leading pipe if one is available. The equivalent line in the reference `libsyntax` is in [`parse_arm`](441519536c/src/libsyntax/parse/parser.rs (L3552)).
As noted in the comment linked above, this feature was formally introduced as a result of rust-lang/rfcs#1925. This feature is in active use in the [`rust-analyzer` codebase](c87fcb4ea5/crates/ra_syntax/src/syntax_kinds/generated.rs (L231))
I have added some tests for this feature, but maybe more would be required
EDIT: Always looking for feedback - is this PR description over-engineered?
Co-authored-by: Daniel McNab <36049421+djmcnab@users.noreply.github.com>
