Merge #1013

1013: Fuzz reparsing and fix found bugs r=matklad a=pcpthm

Add fuzz test for reparsing which:
- Checks reparsing doesn't panic and validate result syntax tree.
- Checks that incremental reparsing produces the same syntax tree as full reparse.
- Check for that errors are the same as full reparsing is disabled because errors are less important than syntax tree and produce failures which I couldn't figure out how to fix immediately (FIXME comment).

I guess the current input generation is inefficient but still found several bugs:
- Arithmetic overflow (negative result on an unsigned type). I changed the signature of `SyntaxError::add_offset` to solve this problem.
- When reparsing a leaf, the token of the leaf can be joined to the next characters. Such case was not considered.
- UNDERSCORE token was not produced when text length is exactly 1 (not a reparsing bug).
- When reparsing a block, *inner* curly braces should be balanced. i.e. `{}{}` is invalid.
- Effects of deleting newlines were not considered.

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

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