expose DR statistics to clients

One driving use case here is wanting the basic block count to evaluate
things like code cache pre-population (xref #2463).

We have dr_stats.h exposed from core/ but it's only used for things built
inside the DR repo: namely, libutil/, linked into the DRstats Windows stats
viewer. dr_stats.h is not exposed publicly for client use. Doing so would
mean it's frozen and we couldn't add stats in the middle anymore. It's
also full of low-level internal DR impl stuff: not really great for public
consumption.

It seems we have two choices:

1) Make dr_stats.h public, but mark it as non-binary-compatible, subject
to change between releases, and full of perhaps hard-to-understand
internal stats.

2) Selectively expose a handful of high-level stats by copying to some new
struct we define in the main client API.