Automatic exponential formatting in Debug
Context: See [this comment from the libs team](https://github.com/rust-lang/rfcs/pull/2729#issuecomment-853454204)
---
Makes `"{:?}"` switch to exponential for floats based on magnitude. The libs team suggested exploring this idea in the discussion thread for RFC rust-lang/rfcs#2729. (**note:** this is **not** an implementation of the RFC; it is an implementation of one of the alternatives)
Thresholds chosen were 1e-4 and 1e16. Justification described [here](https://github.com/rust-lang/rfcs/pull/2729#issuecomment-864482954).
**This will require a crater run.**
---
As mentioned in the commit message of 8731d4dfb4, this behavior will not apply when a precision is supplied, because I wanted to preserve the following existing and useful behavior of `{:.PREC?}` (which recursively applies `{:.PREC}` to floats in a struct):
```rust
assert_eq!(
format!("{:.2?}", [100.0, 0.000004]),
"[100.00, 0.00]",
)
```
I looked around and am not sure where there are any tests that actually use this in the test suite, though?
All things considered, I'm surprised that this change did not seem to break even a single existing test in `x.py test --stage 2`. (even when I tried a smaller threshold of 1e6)
* {:.PREC?} already had legitimately useful behavior (recursive formatting of structs using
fixed precision for floats) and I suspect that changes to the output there would be unwelcome.
(besides, precision introduces sinister edge cases where a number can be rounded up to one
of the thresholds)
Thus, the new behavior of Debug is, "dynamically switch to exponential, but only if there's
no precision."
* This could not be implemented in terms of float_to_decimal_common without repeating the branch
on precision, so 'float_to_general_debug' is a new function. The name is '_debug' instead of
'_common' because the considerations in the previous bullet make this logic pretty specific
to Debug.
* 'float_to_decimal_common' is now only used by Display, so I inlined the min_precision argument
and renamed the function accordingly.
