doc/runtime.tex, lib/sod.3: Restructure the runtime library reference.

Reorder the descriptions of the various macros and functions along more
useful thematic lines.  Some of the sections are currently a bit thin
but they'll fill out later.

Mostly this is just moving text about, but some edits have been made.
Most significantly, `SOD_CONVERT' and `sod_convert' are now discussed
together rather than separately.