Update error messages
This patch restructures how we emit the effect errors for missing
awaits. As we're traversing the expressions, we collect all of the
locations where a missing await is, and metadata about the reason for
the error, as well as the "anchor".
The anchor is where the await should actually be placed in the
expression to fix things. The error is emitted for the whole
sub-expression instead of for the exact synchronization/call-site of the
async function. This avoids the erroneous message for
```
_ = 32 + asyncFunc()
```
The old behaviour was to put the `await` between the `+` operator and
the call to `asyncFunc()`, but this is not a valid place to put effects.
Note; this issue is also present in the fix-it for `try` insertions and
also needs to be fixed. Instead, the anchor points between the
assignment expression and 32, labelling the whole right-side of the
assignment as being the asynchronous function. We emit notes for each
uncovered async point in the entire expression though, so that someone
can see why the expression is async.
