AstGen: rework multi-object for loop

* Allow unbounded looping.
* Lower by incrementing raw pointers for each iterable rather than
  incrementing a single index variable. This elides safety checks
  without any analysis required thanks to the length assertion and
  lowers to decent machine code even in debug builds.
  - An "end" value is selected, prioritizing a counter if possible,
    falling back to a runtime calculation of ptr+len on a slice input.
* Specialize on the pattern `0..`, avoiding an unnecessary subtraction
  instruction being emitted.
* Add the `for_check_lens` ZIR instruction.

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