Optimizations and bug fixes during integration

Introduced a type, exometer_proc, which uses raw message passing
to talk to a metric process. Converted spiral, histogram and netlink
to this type.

Increased min_heap_size to 40000 words. This should be made configurable.
Changed probe process priority from high to normal.

Fixed datapoint handling in exometer_folsom.

Search for predefined template is simpler and quicker.

Inlining used in spiral, histogram, and exometer.

The slot_slide module uses callback funs instead of MFAs (TODO: document)
Optimizations, cleanup, added options

- create timestamp at call time for histograms
- module merging via igor parse_transform
- made histogram and spiral into exometer_proc behavior
- process_flag options for probes
- 'truncate' option for histograms (including folsom module)