* cformat.rs: refactor fill_string to take fill_with_precision
This allows it to be used with both self.min_field_width and
self.precision, which is necessary for padding out %ds with precision.
* cformat.rs: zero-pad %d entries using precision
This matches CPython's behaviour.
* cformat.rs: don't left-adjust when filling with precision
That will always be prepending 0s to %d arguments, the LEFT_ADJUST flag
will be used by a later call to `fill_string` with the 0-filled string
as the `string` param.
* floats: handle alternate form of general formatting
* cformat.rs: convert width/precision to i32
In CPython, width can be isize but precision can only be i32. Our
implementation currently assumes the same type for both: as CPython's
tests assert on overflows for precision, but not for width, we use that
size for both.
* test_format: run test_common_format
Except for the line which raises an OverflowError in CPython, because
overflows in Rust (and therefore RustPython) abort the process
immediately.
* test_types: don't expect test_float_to_string to fail
Its string formatting usage now works as expected.
