pdl: add new ‘Chunk’ struct

This requires more spurious ‘ast::Field’ -> ‘Field’ conversions, which
we’ll clean up next.

Test: atest pdl_tests pdl_inline_tests
(cherry picked from https://android-review.googlesource.com/q/commit:f6a23743e97f732dac4b32a6b8af1f9c71366c46)
Merged-In: I5f639a94e164696c311edbb54b77c10135f3a043
Change-Id: I5f639a94e164696c311edbb54b77c10135f3a043