Collation roclet should use custom parsing

[hadley]

So that the code doesn't need to be source and run, and it can be run quickly and often.

[hadley]

• for each file in R/, readLines, then extract lines that match "^\s*#+' ? @include" (use scan(sep = " ") to split into files)
• (remove all files matched by .Rbuildignore?)
• if no matches, return
• otherwise perform topological sort using wikipedia pseudocode