Basic struct tags support: skip field, assign field

[mtfelian]

Introduced basic struct tags support.

• Use tag deepcopy:"-" to skip copying of a struct field (will be zero-valued);
• Use tag deepcopy:"=" to assign the field, it is specifically needed when you don't want to deep copy a pointer something like *os.File in the struct field.

[dolmen]

The last commit (Converted to module-aware) is irrelevant to this PR.