Add examples and test for type signatures on parametrized productions

[harpocrates]

The current user manual (for version 1.19.12) has this to say about parametrized productions

A drawback of the current implementation is that it does not support type signatures for the parameterized productions, that depend on the types of the parameters. We plan to implement that in the future---the current workaround is to omit the type signatures for such rules.

This is flat out wrong. Parametrized productions have (for at least several versions) supported type signatures. Examples:

many(a) :: { [a] }
  :                        { [] }
  | some_rev(a)            { reverse $1 }

some_rev(a) :: { [a] }
  :  a                     { [$1] }
  | some_rev(a) a          { $2 : $1 }

then(a, b) :: { (a,b) }
  : a b                    { ($1, $2) }

To fix:

• add a regression test case to make sure this is not broken in future versions
• add an example to examples for this
• update the user manual

This is somewhat important since there is also bad advice floating about the internet suggesting people use _ to work around this (see #141 for instance).