Simplify FixedDecimal's rounding APIs (#5028)

Related to #2902

Mainly removes all the `_to_increment` methods, which are superseeded by
the `round_with_mode_and_increment` method.

Additionally, adds a `round` function that does half to even rounding.

components/experimental/src/compactdecimal/compactdecimal.rs

@@ -490,11 +490,11 @@ impl CompactDecimalFormatter {
  // If we have just one digit before the decimal point…
  if significand.nonzero_magnitude_start() == 0 {
  // …round to one fractional digit…
- significand.half_even(-1);
+ significand.round(-1);
  } else {
  // …otherwise, we have at least 2 digits before the decimal point,
  // so round to eliminate the fractional part.
- significand.half_even(0);
+ significand.round(0);
  }
  let rounded_magnitude = significand.nonzero_magnitude_start() + i16::from(exponent);
  if rounded_magnitude > log10_type {