Ease restrictions on self-call

[iamdefinitelyahuman]

What I did

Ease restrictions on a contract calling into itself:

• remove the runtime assertion
• remove the compile-time check on an interface cast to self

Closes #1696

How I did it

• Deleted a couple of checks in vyper/parser/external_call.py
• Minor documentation update - we never really explicitly said this didn't work, so there wasn't much to modify here. I could write more, but.. I'll do a proper documentation update soon:tm:, promise

How to verify it

Run the tests. I flipped a few tests around that were verifying this behavior wasn't working, now they check that it works.

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2059 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2059      +/-   ##
==========================================
- Coverage   85.06%   85.06%   -0.01%     
==========================================
  Files          83       83              
  Lines        8115     8113       -2     
  Branches     1957     1956       -1     
==========================================
- Hits         6903     6901       -2     
  Misses        730      730              
  Partials      482      482              

Impacted Files	Coverage Δ
vyper/parser/external_call.py	80.59% <ø> (-0.57%)	⬇️

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