Feature request: Show usages/references to types.

[PiotrZSL]

Classes that have default constructor, C structs have no usages/references in Woboq.
Woboq should show also references to such compiler generated methods.

Could look like this:
Moving mouse above class/struct should show as usages:

• construction of type (call to constructor in any way)
• references to type
• list of all variables that use this type.