internal: pin_data: refactor for tuple struct support

Introduce `FieldInfo` struct to encapsulate field and other relevant
data (e.g. pinned and member name) to abstract over named/unnamed
fields and extract relevant field data code into methods.

Also, generate projections and pin-data accessors for unnamed
members.

Signed-off-by: Mohamad Alsadhan <mo@sdhn.cc>

Author: mqqz