Add CompilationUnit.implementation_address when the CU is a proxy contract

[SheldonHolmgren]

This is a small step towards #132. Instead of following the proxy within crytic-compile, this PR allows applications built on top of crytic-compile or slither to do that themselves.

[SheldonHolmgren]

Tested with ipython:

In [1]: from crytic_compile import CryticCompile                                                                                                                                                                         
                                                                                                            
In [2]: print(list(CryticCompile("arbi:0x5A572B5fBBC43387B5eF8de2C4728A4108ef24a6").compilation_units.values())[0].implementation_address)
arbi:0xbca6d5de39c50a863d603702101ab776ce525c3a       

In [3]: print(list(CryticCompile("arbi:0xbca6d5de39c50a863d603702101ab776ce525c3a").compilation_units.values())[0].implementation_address)
None

In [5]: print(list(CryticCompile("0x5e4861a80B55f035D899f66772117F00FA0E8e7B").compilation_units.values())[0].implementation_address)
0x8d014903bf7867260584d714e11809fea5293234

In [6]: print(list(CryticCompile("0x8d014903bf7867260584d714e11809fea5293234").compilation_units.values())[0].implementation_address)
None

(I've cut out the noise in the form of compilation warnings.)

[SheldonHolmgren]

The only test failure is:

Etherscan #6
  ERROR:CryticCompile:Etherscan API rate limit exceeded
  ERROR:CryticCompile:Etherscan API rate limit exceeded
  Etherscan #6 test failed

but my change doesn't increase the number of API requests so I believe this failure is unrelated.

[montyly]

Thanks @SheldonHolmgren