main.go: Rename Meta* objects (types, funcs and vars) to IR or MIR names

[DQNEO]

I named middle layer objects "Meta" because originally they were additional information to AST objects, but over time they grew bigger and now they are not only "additional" stuff. Actually they are main objects that the code generator is directly dealing with.

New names can be "IRXxx" or "MIRXxxx"

[DQNEO]

This kind of massive renaming would cause a large amount of conflicts, so we should take care of other big ongoing branches.