[FEAT] Merge IRModules

[tqchen]

Now we finished the first step of global symbol linkage, we are no unblocked to add a feature to merge IRModules. The main thing to address during the merge is name conflict. If both module a and module b have a function with the same name but different semantics, we need to be able to rename them when they are private:

Possible sketchs of actions

• Add a rename pass that renames functions (optionally rename non-private ones)
  • Take a rename map (GVar=>String or String=>String)
  • Take an bool option to rename_external_funcs (default to false) as we want to avoid doing so unless user really want to do so
  • When rename_external_funcs is false, forcing rename of external func(those with global symbol) causes an error.
• Scan and find create rename map (GVar=>GVar)
• Report error when there is conflicting global var that is also external (per structural equality)
• Run rename pass and merge.

[slyubomirsky]

We could perhaps allow users to specify some renaming convention so that any name changes would be predictable.

[tqchen]

agree, for private functions, the renaming can happen through adding numbering suffix. By default we should not rename external functions. Because the users only interact with external functions, in theory they should not care about private function renaming but agree having a simple predicatble rule would help