box: introduce tuple_field_raw_by_path routine

Introduced a new function tuple_field_raw_by_path is used to get
tuple fields by field index and relative JSON path. This routine
uses tuple_format's field_map if possible. It will be further
extended to use JSON indexes.
The old tuple_field_raw_by_path routine used to work with full
JSON paths, renamed tuple_field_raw_by_full_path. It's return
value type is changed to const char * because the other similar
functions tuple_field_raw and tuple_field_by_part_raw use this
convention.
Got rid of reporting error position for 'invalid JSON path' error
in lbox_tuple_field_by_path because we can't extend other
routines to behave such way that makes an API inconsistent,
moreover such error are useless and confusing.

Needed for #1012