Support TOML format for parameters

[rcoreilly]

TOML is much cleaner & human friendly than the go native literal initializer or json -- can just put a big string in code and read from that instead of the literal initializer, and /or read from files.

[rcoreilly]

It works great as a file format, where you get nice highlighting and formatting help from the editor -- but not so great as a string blob in a .go file, where you don't. Anyway, now available at any level for read / write.

had to change top-level Sets to be a map instead of an array, because toml doesn't marshal top-level arrays... This requires a stuttering addition of Set name in map initializers.