feat[lang]: implement function exports

[charles-cooper]

What I did

implement exports: ...

todo:

• update validate_implements
• add tests
• ban external functions from self.

How I did it

How to verify it

Commit message

add `exports:` declaration to the language. this allows users to
directly export functions marked `@external` from libraries instead of
writing `@external` wrapper functions (and consequently, for library
authors to define the external interface for their modules).

important refactors:
- remove expanded getters from module AST, change them to annotation on
  public VariablDecls.
- redo order of node visitation in ModuleAnalyzer
- refactor `InterfaceT.validate_implements()` to handle exported
  functions
- add `exposed_functions` property to `ModuleT` which reflects the
  runtime functions exposed in the selector table
- refactor IR generation to use new reachability analysis

misc:
- remove Module.add_to_body, Module.remove_from_body as they are dead
  now
- move VariableDecl validation to vyper/ast/nodes.py
- improve call-site annotations for error messages
- improve annotations for exceptions which reference a previously-
  declared node.

Description for the changelog

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.74074% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 85.08%. Comparing base (9bab114) to head (4b91ac1).

Files	Patch %	Lines
vyper/exceptions.py	70.27%	8 Missing and 3 partials ⚠️
vyper/semantics/analysis/module.py	93.02%	3 Missing and 3 partials ⚠️
vyper/semantics/types/function.py	76.92%	5 Missing and 1 partial ⚠️
vyper/semantics/types/module.py	92.85%	3 Missing ⚠️
vyper/ast/nodes.py	91.30%	1 Missing and 1 partial ⚠️
vyper/semantics/analysis/base.py	91.66%	1 Missing ⚠️
vyper/semantics/types/user.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3786      +/-   ##
==========================================
+ Coverage   85.02%   85.08%   +0.05%     
==========================================
  Files          92       92              
  Lines       13767    13875     +108     
  Branches     3082     3113      +31     
==========================================
+ Hits        11706    11805      +99     
- Misses       1572     1576       +4     
- Partials      489      494       +5     

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