Leftry

This library is for creating and composing parsers.

For example:

```
local grammar = require("leftry")
local factor = grammar.factor
local span = grammar.span
local A = factor("A", function(A) return
span(A, "1"), "1"
end)
local B = factor("B", function(B) return
span(B, "2"), A
end)
print(B("111122222", 1))
```

This creates a parser `B` that can parse the string "111122222".