Handling streaming input over stdin

[tsnl]

I like using sexpfmt to format log messages. Unfortunately, without this PR, sexpfmt will wait for EOF before spitting out formatted output. This PR is intended to improve this situation.

Parsing large input is more cache-efficient when lexing and parsing occur in separate passes. Parser and lexer generators like lex and yacc (or their modern counterparts, flex and bison) typically accept input character-at-a-time. However, more modern parser frameworks, like LALRPOP and nom (the latter used in this project at the moment) rely on string input to lex and parse in separate passes.

As a compromise, we read input using the 'matching parentheses' property of S-expressions, using spaces to delimit atoms and parentheses (as well as other bookends) to delimit lists. This way, we can read each form into a temporary buffer, parse, and then print. This branch implements this approach. It is not as elegant as the Yacc-like approach since we must effectively lex twice, but will do for now and should be functionally equivalent.