This adds the infrastructure for the "field cache", which is just
various helpers for the dc_get_field() interface.
This includes the 'dc_field_cache_t' structure that a libdivecomputer
backend can just add to its parser data structure, and a few macros to
make it very easy to initialize the fields and then return them in the
'get_field()' callback.
And part of it is the infrastructure support for the 'dc_field_string_t'
type, which adds the support for string fields. That will be used to
return various string-formatted data from the dive computer, like deco
models, serial numbers, etc.
And no, a serial number is most definitely not a "number". It's a string.
Right now there are no users of this yet, that comes next.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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