text: Title: Use function summaries instead of inlining 
  Body: We [currently use inlining instead of summaries](https://github.com/python-security/pyt/tree/master/pyt/cfg), for inter-procedural analysis, which makes PyT slower than it needs to be.
  
  Here are some videos, specifically the last one, explains function summaries well:
  [#57 Call Graphs](https://www.youtube.com/watch?v=giGqdwuZBKQ)
  [#58 Interprocedural Data Flow Analysis](https://www.youtube.com/watch?v=TJA7dvAV0ZI)
  [#59 Procedure Summaries in Data Flow Analysis](https://www.youtube.com/watch?v=LrbPmaLEbwM)