implement codefix for C# lambda => F# lambda

[baronfel]

when passing a C# lambda (wrapped in parens), the compiler sees it as (roughly):

SynExpr.Paren(
  SynExpr.App(
    SynExpr.App (
      Ident "op_EqualsGreater",
      <expr for lambda args>
    ),
    <expr for lambda body>
)

so we can extract that pattern out and rewrite it.

This handles the following cases:

• someFunc (thing => otherthing)
• let thing = args => body

[baronfel]

reporting of this error is much better now. I'd love to hear about more scenarios where we can detect this.

[baronfel]

I freaking love active patterns

[baronfel]

so fun fact: this is the shape of this erroneous usage, so any more cases we want to cover should just be a matter of plugging this into the appropriate expr match.

[Zaid-Ajaj]

Without active patterns, matching the F# AST would be so much harder!