Show fully-qualified name in hover text

[larsk21]

Show the fully-qualified name of a declaration when hovering over it. unresolveNameGlobal is changed to return the fully-qualified name when pp.fullNames is set to true, which is the description of this option.

The type shown in the hover is not changed.

semi-closes #749

[Kha]

Here is a potential issue with the new pp code:

constant foo : Nat

namespace Bar

constant foo : Nat

set_option pp.fullNames true
#check _root_.foo  -- should print `_root_.foo`

end Bar

I think the cleanest solution is to extend your code and duplicate a bit of logic from unresolveNameCore: if pp.fullNames is set, check if resolveGlobalName n₀ resolves to n₀. If it doesn't, prepend rootNamespace.