add text data property test.

Found a few bugs already that the previous eunit tests hadn't uncovered..!

Added dbg make target, to speed up debugging by booting up the debugger and breaking in a property test.
clean up property types.

This also gives a better spread on data and pointer section sizes, as the ?SIZED macro only used a single size counter..
more property type refactorings..
clean up prop test initialization code
test data fields with schema.
add remaining capnp value types.
add more comments to spare my sanity in the future..
ptr field prop test.

Data alloc now supports adding new segments,
and ecapnp_ref:alloc_data supports far pointers.

go proper!
more lists of more primitive types..
refactor field access property tests.
add text type to ptr field test.

add code for missing far ref in write_data.
refactor write_data.
replace get_/set_segment with read/write.
test list of texts.
add missing pieces for commit 2f7e2e42db51c186a16ae14ab1e61b67464ada5c
add new compiled schema.
wip: reorganizing schema lookups..
adapt to new schema stuff..
get the sample working..
update readme and add samples target to makefile.