Speed up map creation.

- Rewrite the Maps::Parse to use open, and a buffer on the stack.
- Rewrite the line parser away from sscanf. The current way sscanf
  is used does not catch many malformed lines. In addition, this
  new version improves performance by 50% over sscanf on sailfish.
- Add a lot of unit tests for the parser to make sure there are
  no problems. In addition, add a special line that was not rejected
  with the previous version of the code.
- Add new accessor to get the map at a particular index.
- Add a backtrace benchmark for map creation for both new and old.

This cl results in ~5% speedup and makes the new unwinder map creation
about the same for 64 bit. It's still a bit slower, but not by much.
On 32 bit, we are still about 5% slower than the old creation method,
though.

Bug: 23762183

Test: libunwindstack unit tests pass. Ran the new benchmarks.
Change-Id: Id4431e539f400984e6fad62153fdf4152d518322