Change how the `0` flag works in format!
Now it always implies right-alignment, so that padding zeroes are placed after the sign (if any) and before the digits. In other words, it always takes precedence over explicitly specified `[[fill]align]`. This also affects the '#' flag: zeroes are placed after the prefix (0b, 0o, 0x) and before the digits.
Here's a short summary of how similar format strings work in Python and Rust:
```
:05 :<05 :>05 :^05
Python 3.6 |-0001| |-1000| |000-1| |0-100|
Rust before |-0001| |-1000| |-0001| |-0100|
Rust after |-0001| |-0001| |-0001| |-0001|
:#05x :<#05x :>#05x :^#05x
Python 3.6 |0x001| |0x100| |000x1| |00x10|
Rust before |0x001| |0x100| |000x1| |0x010|
Rust after |0x001| |0x001| |0x001| |0x001|
```
Fixes#39997 [breaking-change]
e5221f9397