Merge #996

996: Allow attributes on top level expressions r=matklad a=pcpthm

This PR modifies parser to allow outer attributes on top level expression. Here, top level expression means either
- Expression statement e.g. `foo();`
- Last expression in a block without semicolon `bar()` in `{ foo(); bar() }`.

Except for binary operation expressions and `if` expressions, which are errors (feature gated) in rustc.
Attributes on inner expressions like `foo(#[a] 1)` are not implemented.

I first tried to implement this by passing `Maker` to expression parsers. However, this implementation couldn't parse `#[attr] foo()` correctly as `CallExpr(Attr(..), PathExpr(..), ArgList(..))` and instead parsed incorrectly as `CallExpr(PathExpr(Attr(..), ..), ArgList(..))` due to the way left recursion is handled.
In the end, I introduce `undo_completion` method. Which is not the suggested approach, but it seems not very bad.

Fix #759.


Co-authored-by: pcpthm <[email protected]>

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