Add parsing and writing

[yitzchak]

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[yitzchak]

@paulapatience There are few issues left to iron out here in addition to whatever results from your review.

First, the parse-digits, read-digits, compose-digits and write-digits are all currently generic, but I am wondering if we should remove the client and make those regular exported "utility" functions since the individual number parsers call those with various parameters depending on the parsing branch.

Second, the write-number and compose-number interface is bogus. I haven't worked on that yet so it is probably not worth reviewing yet since I'll probably revamp all of it. I'm not sure I'll be able to get there immediately, but I'd like to have a low-level generic write-number/compose-number and a high level function in quaviver/string that is basically equivalent to FORMAT but with C, CL, JSON, etc. clients.