Split the license file into two, one for the main sources and one for Go
language derived sources.
- Make speakc read files instead of lexing a hard-coded string.
- Added new choice contruct, to lexer and spec.
- Added some test data.
cmd/speakc => speakc
Use exported names in lex.go for things that are supposed to be exported.

lex.go is in package main, this change prepares for a move to another package name.
Created parser and moved code from main to it.
Split ParseFile into ParseFile and ParseText.
spec: Enumeration value range and choice grammar.