PPX syntax for tyre regular expressions and routes

This PPX compiles

[%tyre {|re|}]

into 'a Tyre.t and

function%tyre
| {|re1|} as x1 -> e1
...
| {|reN|} as x2 -> eN

into 'a Type.route, where re, re1, ... are regular expressions expressed in a slightly extended subset of PCRE. The interpretations are:

• re? extracts an option of what re extracts.
• re+, re*, re{n,m} extracts a list of what re extracts.
• (?@qname) refers to any identifier bound to a typed regular expression of type 'a Tyre.t.
• One or more (?<v>re) at the top level can be used to bind variables instead of as ....
• One or more (?<v>re) in a sequence extracts an object where each method v is bound to what re extracts.
• An alternative with one (?<v>re) per branch extracts a polymorphic variant where each constructor `v receives what re extracts as its argument.
• (?&v:qname) is a shortcut for (?<v>(?&qname)).