Reduce default JFlex buffer size from 16k

[cowtowncoder]

Although performance is not the most important aspect of TOML handling, I did some profiling for funsies and noticed that JFLex default buffer size of 16k characters (32kB) is rather expensive, considering it is not recycled.
Since it looks like there is a way to configure that, as per

https://github.com/jflex-de/jflex/blob/master/docs/md/lex-specs.md

(specifically, use %buffer directive)

let's tune it down a bit: most input does not require contiguous buffer of that size, and if something does, Lexer just grows buffer size as necessary.