LambdaMetafactory: standardize supported type conversions

The conversions allowed by LambdaMetafactory on parameter and return types are ad hoc and inconsistent with the specification.

Propose standardizing on the conversions supported by MethodHandle.asType, which are similar but a little more flexible.

Specifically:

factoryType.parameterType(i) --> implementation.type().parameterType(i)
Currently: same type (but receiver can be a subtype)
Change: allow any 'asType' conversion, including boxing & unboxing

dynamicMethodType.parameterType(i) --> implementation.type().parameterType(j)
Currently: ref subtype, prim widening, or box/unbox followed by these (but receiver can't be boxed)
Change: allow conversion between arbitrary refs; allow narrowing from a supertype before unbox; allow receiver to be boxed

interfaceMethodType.parameterType(i) --> dynamicMethodType.parameterType(i)
Currently: ref supertype, prim same
Change: allow any 'asType' conversion, including boxing & unboxing

implementation.type().returnType() --> dynamicMethodType.returnType()
Currently: any type targeting void, arbitrary refs, prim widening, box+widening, unbox+widening*
Change: allow void to target a non-void type; allow narrowing from a supertype before unbox

dynamicMethodType.returnType() --> interfaceMethodType.returnType()
Currently: void->void, ref subtype, prim widening, box+widening, unbox+widening
Change: allow either to be void; allow conversion between arbitrary refs; allow narrowing from a supertype before unbox

(*Current implementation allows any reference-typed implementation return to be compatible with any expected primitive type, as long as the reference type is not a box type (if it is a box type, unbox+widening applies). This is probably a bug.)