Finally shut up the last 3 compile warnings.

in valgrind_errors().
Speed up reading a DWARF entry

- inline the setup() function
- pass the value_t to add_attribute by reference
- rejig attribute storage to trade off space vs instructions

Results: np_init() in the tnapass test program falls from 1.8 Ginsn
to 254 Minsn as measured by callgrind.
Speed up DWARF scanning a bit more

By adding a filtering option to walker_t which allows us to skip reading
the attributes of entries which aren't the tag we're looking for.  This
shaves the tnapass np_init() down to 229 Minsn.
Add a simple profiling mechanism

which makes it possible to get some simple profile data out when running
the program under Valgrind, because Valgrind doesn't play well with
existing profile tools, and performance under Valgrind is actually the
metric we want to measure.