#936, #935, #934, #933, #932, #931)

* Allow walrus in slices

See python/cpython#23317

Raised in #930.

* Fix parsing of nested f-string specifiers

For an expression like `f"{one:{two:}{three}}"`, `three` is not in an f-string spec, and should be tokenized accordingly.

This PR fixes the `format_spec_count` bookkeeping in the tokenizer, so it properly decrements it when a closing `}` is encountered but only if the `}` closes a format_spec.

Reported in #930.

* Fix tokenizing `0else`

This is an obscure one.

`_ if 0else _` failed to parse with some very weird errors. It turns out that the tokenizer tries to parse `0else` as a single number, but when it encounters `l` it realizes it can't be a single number and it backtracks.

Unfortunately the backtracking logic was broken, and it failed to correctly backtrack one of the offsets used for whitespace parsing (the byte offset since the start of the line). This caused whitespace nodes to refer to incorrect parts of the input text, eventually resulting in the above behavior.

This PR fixes the bookkeeping when the tokenizer backtracks.

Reported in #930.

* Allow no whitespace between lambda keyword and params in certain cases

Python accepts code where `lambda` follows a `*`, so this PR relaxes validation rules for Lambdas.

Raised in #930.

* Allow any expression in comprehensions' evaluated expression


This PR relaxes the accepted types for the `elt` field in `ListComp`, `SetComp`, and `GenExp`, as well as the `key` and `value` fields in `DictComp`.

Fixes #500.

* Allow no space around an ifexp in certain cases

For example in `_ if _ else""if _ else _`.

Raised in #930. Also fixes #854.

* Allow no spaces after `as` in a contextmanager in certain cases

Like in `with foo()as():pass`

Raised in #930.

* Allow no spaces around walrus in certain cases

Like in `[_:=''for _ in _]`

Raised in #930.

* Allow no whitespace after lambda body in certain cases

Like in `[lambda:()for _ in _]`

Reported in #930.