Allow STATICCALL using raw_call

[iamdefinitelyahuman]

What I did

Add is_static_call kwarg to raw_call as a way to perform a STATICCALL.

Closes #1971

How I did it

Minor tweaks to the RawCall.build_LLL logic in vyper/functions/functions.py:

• use of is_static_call and is_delegate_call together raises
• raw_call may now be used in @constant functions, ONLY when is_static_call=True. If the kwarg is omitted, the exception message suggests it's use to the user.

How to verify it

Run the tests. I added a few cases to verify that it works.

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[codecov-io]

Codecov Report

Merging #1973 into master will increase coverage by 0.00%.
The diff coverage is 77.77%.

@@           Coverage Diff           @@
##           master    #1973   +/-   ##
=======================================
  Coverage   87.16%   87.17%           
=======================================
  Files          58       58           
  Lines        6631     6636    +5     
  Branches     1674     1677    +3     
=======================================
+ Hits         5780     5785    +5     
  Misses        536      536           
  Partials      315      315           

Impacted Files	Coverage Δ
vyper/functions/functions.py	92.33% <77.77%> (+0.06%)	⬆️

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