Move reserved keywords check into the type checker

[iamdefinitelyahuman]

What I did

Move reserved keyword checks into the type checker.

How I did it

The logic was already partially being handled within type checking: shadowing of builtins and environment variables would raise upon attempted assignment. However, reserved keywords and opcodes weren't caught until later when generating the GlobalContext object.

I've moved the reserved keywords list into namespace and am now applying the check on all new assignments as they're added to the Namespace object. For object members (values in events, structs, interfaces) I added a validate_identifier method that's slightly looser: it doesn't raise on collisions between non-builtins, so that e.g MyStruct.foo and self.foo won't collide.

How to verify it

Run the tests. I had to modify a few tests in order to make this work, because the exception being raised in some places has changed.

Cute Animal Picture

[codecov-io]

Codecov Report

Merging #2286 (43d8305) into master (69f0402) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2286      +/-   ##
==========================================
+ Coverage   85.45%   85.47%   +0.01%     
==========================================
  Files          84       84              
  Lines        8583     8580       -3     
  Branches     2076     2079       +3     
==========================================
- Hits         7335     7334       -1     
+ Misses        747      746       -1     
+ Partials      501      500       -1     

Impacted Files	Coverage Δ
vyper/parser/context.py	95.29% <ø> (-0.17%)	⬇️
vyper/parser/expr.py	77.77% <ø> (-0.05%)	⬇️
vyper/parser/global_context.py	77.77% <ø> (ø)
vyper/context/namespace.py	96.87% <100.00%> (+0.57%)	⬆️
vyper/context/types/meta/event.py	77.58% <100.00%> (+1.22%)	⬆️
vyper/context/types/meta/interface.py	81.48% <100.00%> (+0.34%)	⬆️
vyper/context/types/meta/struct.py	80.00% <100.00%> (+0.89%)	⬆️
vyper/signatures/event_signature.py	94.44% <100.00%> (-0.43%)	⬇️
vyper/signatures/function_signature.py	77.16% <100.00%> (-0.56%)	⬇️
vyper/types/types.py	82.41% <100.00%> (-0.09%)	⬇️

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[fubuloubu]

it would be lovely if we can somehow pull this from our stuff dyanmically, instead of statically maintaining this alongside changes to the compiler

[iamdefinitelyahuman]

Definitely worth implementing.