Treat `Function` literals uniformly, expecting SAM or FunctionN.

They both compile to INDY/MetaLambdaFactory, except when they
occur in a constructor call. (TODO: can we lift the ctor arg
expression to a method and avoid statically synthesizing
anonymous subclass altogether?)

Typers:
  - no longer synthesize SAMs -- *adapt* a Function literal
    to the expected (SAM/FunctionN) type
  - Deal with polymorphic/existential sams (relevant tests:
    pos/t8310, pos/t5099.scala, pos/t4869.scala) We know where
    to find the result type, as all Function nodes have a
    FunctionN-shaped type during erasure. (Including function
    literals targeting a SAM type -- the sam type is tracked as
    the *expected* type.)

    Lift restriction on sam types being class types. It's enough
    that they dealias to one, like regular instance creation
    expressions.

Contexts:
  - No longer need encl method hack for return in sam.

Erasure:
  - erasure preserves SAM type for function nodes
  - Normalize sam to erased function type during erasure,
    otherwise we may box the function body from `$anonfun(args)`
    to `{$anonfun(args); ()}` because the expected type for the
    body is now `Object`, and thus `Unit` does not conform.

Delambdafy:
  - must set static flag before calling createBoxingBridgeMethod
  - Refactored `createBoxingBridgeMethod` to wrap my head around
    boxing, reworked it to generalize from FunctionN's boxing
    needs to arbitrary LMF targets.

Other refactorings: ThisReferringMethodsTraverser, TreeGen.

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