this commit removes `asttokens` from the parse machinery, since the
method is buggy (see below bugs) and slow. this commit brings down
parse time (time spent in ast generation) between 40-70%.

the `mark_tokens()` machinery is replaced with a modified version of
`python.ast`'s `fix_missing_locations()` function, which recurses
through the AST and adds missing line info based on the parent node.

it also changes to a more consistent method for updating source
offsets that are modified by the `pre_parse` step, which fixes several
outstanding bugs with source location reporting.

there were some exceptions to the line info fixup working, the issues
and corresponding workarounds are described as follows:

- some python AST nodes returned by `ast.parse()` are singletons, which
  we work around by deepcopying the AST before operating on it.

- notably, there is an interaction between our AST annotation and
  `coverage.py` in the case of `USub`. in this commit we paper over the
  issue by simply always overriding line info for `USub` nodes. in the
  future, we should refactor `VyperNode` generation by bypassing the
  python AST annotation step entirely, which is a more proper fix to the
  problems encountered in this PR.

the `asttokens` package is not removed entirely since it still has a
limited usage inside of the natspec parser. we could remove it in a
future PR; for now it is out-of-scope.

referenced bugs:
- https://github.com/vyperlang/vyper/issues/2258
- https://github.com/vyperlang/vyper/issues/3059
- https://github.com/vyperlang/vyper/issues/3430
- https://github.com/vyperlang/vyper/issues/4139