Rename future-impl.h to future-internal.h.
threads.h: Fix some internal documentation.
compiler.h: Fix extra semicolon in macro.
future: Add a race-free way to examine a partial computation.

This allows safe OpenGL previews, for example.  dmnsn_future* learned
the dmnsn_future_{pause,resume}() functions which cause all worker
threads to block.  render.test now survives Helgrind with no errors.