pdl: support scalar fields with arbitrary widths

Before we only supported fields with integer byte widths (8, 16, 32,
24, … bits). We now support all integer bit widths between 1 and 64.

This required a complete rewrite the parsing and serializaton logic
since we need to chunk bit fields until we reach a byte boundary.

Test: atest pdl_inline_tests
Change-Id: Icec8f2155759c572a3445a7e81515337cc3660e4