Main namespace for building and using Parka's PEG parsers.
The functions in this file create Parka structures to describe, for example,
a choice between several expressions.
Such an expression can be compiled into a parser engine, and then this engine
can be executed repeatedly on input strings.

Parser expressions are constructed from the following:
- Characters parse exactly themselves.
- Strings parse as a sequence of characters, exactly as given.
- Sets of Characters parse any one character which is a member of the set.
- A vector is a nestable sequence (equivalent to calling `seq`).
- Keywords reference a nonterminal in the same grammar.
- Each of the parser combinators in this namespace produces an expression.

A grammar is a map from keyword labels to parsing expressions, plus a start
symbol. Grammars are themselves a parsing expression and can be nested.

Once you have constructured your complete expression, call `compile` with it.
The returned engine can be run by calling `parse`.

Handles the compilation from the PEG grammar syntax to the parsing machine
instructions.

Helper stream library for parsing, they're maps, like this:

| key | description |
| --- | ----------- |
| `:str` | Entire input string |
| `:pos` | Current index into that string |
| `:filename` | Source file name |
| `:line` | Current line number (1-based) |
| `:col` | Current column number (0-based) |
| `:value` | Arbitrary parsing result (optional) |
| `:state` | Arbitrary user state (optional) |