A "usually" allocation-free cling::Value (ROOT-6075).
Replace StoredValueRef by Value.
Unused extern decl.
Simplify; adapt to new Value.
Set the QualType for enums, too; return 0 for cast-from-void.
Default init TInterpreterValue in TClingValue copy ctor. Wording.
Implement cheap move ctor/op=. Member-init order.
No vector for time-critical i'faces; use SmallVectorImpl in libCling i'faces.

The use of vector forces an allocation for each call, something we don't need to
impose.

SmallVectorImpl doesn't expose the static alloc size; and our interfaces don't
care. Only the actual storage definition needs to define the size, no subsequent
reference.
Simplify and accelerate CallFunc, cling::Value (ROOT-6075).

Brings PyROOT::TMethodHolder::Execute() from 2.7M down to 1.4M instructions,
according to callgrind and for the test case of ROOT-6075.