Fix several cppcheck warnings.
Fix first-time make config=coverage test

Not sure why xargs -r is not the default...
XPath: Fix undefined behavior while calling memcpy

Calling memcpy(x, 0, 0) is technically undefined (although it should usually
be a no-op).

Fixes #20.
tests: Improve test coverage
Fix undefined behavior while calling memcpy

Calling memcpy(x, 0, 0) is technically undefined (although it should usually
be a no-op).
tests: Add more XPath mod tests
XPath: Refactor predicate application

Split number/boolean filtering logic into two functions. This creates an
extra copy of a remove_if-like algorithm, but moves the type check out of
the loop and results in better organized filtering code.

Consolidate test-based dispatch into apply_predicate (which is now a member
function).
Merge branch 'master' into compact
Fix gcc warnings in compact mode