Improved top (#37)

* Improved top

1. Keymap - sort, clear, auto-refresh
2. Sort works under point, and interactively. Sorting twice
   under the same column flips the order. Sort works on the
   command column, too.
3. Clear clears the accumulated statistics.
4. Auto-refresh now is requests the right buffers and values
   if called interactively.
5. Change various lists into vectors for performance.
6. Create a new struct to capture command-entry instead of
   nth'ing into a list.
7. Actually make it not typecheck in byte-compiled mode.
8. Add new column to top, "slow calls" which are the number
   of calls ... slow.
9. Fix bugs if user requests the top-major-mode on a new
   buffer, instead of using `explain-pause-top` directly.
A. Add the beginnings of more unit tests.

* Add custom faces. Rewrite table code for better encapsulation.

1. Add "changed", "slow" faces
2. Rewrite the table code so command-entries deal with themselves
   and we never reach inside them from table layer.
3. Improve performance by moving to macros so we get typechecking
   off in bytecode.
4. Make each buffer hold it's own stats, which will be useful
   later for freezing tables.

* Add faces for the header when sorting.

* Split long command lines by spaces. Only cut strings if we have to.