Merge DesugaredLambda and Lambda to one active pattern.

[nojaf]

Fixes #1631.

    | Lambda of
        fromMethod: bool *
        inLambdaSeq: bool *
        args: SynSimplePats *
        body: SynExpr *
        parsedData: (SynPat list * SynExpr) option *
        range: range

parsedData contains all the information necessary to print out the parameters of a lambda (destructed or not).
This simplifies Fantomas a bit like the whole ComplexPats dance can be skipped.