Some headers used macros named like _THING_H for multiple-include
protection. That style of name is reserved in ISO C, though, so I've
replaced it with PUZZLES_THING_H which is my favourite of the other
styles in use.
in the 'unfinished' directory for a while, and has now been finished
up thanks to James Harvey putting in some effort and galvanising me to
put in the rest. This is 'Pearl', an implementation of Nikoli's 'Masyu'.
The code in Loopy that generates a random loop along grid edges to use
as the puzzle solution has been abstracted out into loopgen.[ch] so
that Pearl can use it for its puzzle solutions too. I've also
introduced a new utility module called 'tdq' (for 'to-do queue').
[originally from svn r9379]
