Include batteries

[AndrasKovacs]

The idea is to have a TH-configurable interface that covers a lot of needed features in PL parsing. The inputs could be the following:

• Identifier parsing: first character, rest of characters.
• Whitespace characters.
• Comments, line/multiline, nestedness.
• List of keywords.
• Type of errors, rendering possibly custom or configurable.

Output:

• Identifier parser, keyword parser, whitespace parser, token combinator.
• Functions for error handling.
• switch function that automatically handles keyword/identifier disambiguation and tokenization. For example, if I switch on "let", I don't want to manually cover the case where we actually have an identifier that has "let" as prefix. This should be handled at compile time, by using the user-provided functions for identifier/keyword parsing. The source spans of switched things should be also made accessible in a concise way.

There could be two flavors of batteries, one for Basic without indentation tracking an another for Stateful with it. There should be at least a larger PL benchmark too that uses our batteries. That could also serve as tutorial.

I'm interested in suggestions about what kind of features could be included in the batteries.