feat!: make parsing and formatting no-alloc

[robjtede]

Prepares no-alloc support for the breaking v3 release.

• Adds the alloc feature and a no-alloc compatibility target.
• Keeps parsing and formatting available when all features are disabled.
• Replaces the String error from ByteSize::from_str() with
  ByteSizeParseError.
• Makes ByteSizeParseError and UnitParseError implement
  core::error::Error without allocation.
• Formats width and precision directly through core::fmt, without an
  intermediate String.

The ByteSize::from_str() error type change is the intentional breaking change
for v3. No version files change; release-plz will prepare the release after
merge.

Validated with:

• just check
• just test
• just test-msrv
• just build-no-std
• just build-no-alloc