Range and span expressions

[ptal]

Manipulation of span in parser is something utterly important. We feel the need to propose some support to handle span more easily. We therefore propose two expression extensions:

(... e1 e2)

which have the type (Range<S>, T1, T2) where Range<S> is as defined in #85 and

(.. e1 e2)

with the type (Span, T1, T2) where Span is obtained by calling range.span().

Design

• Add an expression StreamRange(usize) in the AST.
• Add a type Span along with Range.
• Force .. and ... to start a sequence, e1 .. e2 is disallowed because it leads to visual ambiguities.
• We do not propose Range<(T1, T2)> (for example) because we feel it's inconvenient, it forces the user to extract the span and the value in the semantic action.

It was first proposed in #13 to infer span, we propose here a better control while the span type is still correctly inferred and propagated.

[ptal]

Triage of issues: I abandon the syntax ... (only .. remains) for now. It is subsumed by a more general mechanism which allows us to call external parsers inside a Oak rule.