kernel_cmdline: Refactor into separate bytes and utf8 modules

Split the kernel command line parsing functionality into two focused
modules. The `bytes` module handles raw byte parsing without UTF-8
requirements, matching kernel behavior for arbitrary byte
sequences. The `utf8` module provides string-based parsing for cases
where UTF-8 validation is needed.  The `utf8` module reuses the
`bytes` module primitives where possible, and uses the fact that
`utf8::Cmdline` can only be constructed from valid UTF-8 to do
unchecked conversions between the two.

Signed-off-by: John Eckersberg <[email protected]>

Author: jeckersb