fix[lang]: fix encoding of string literals

[charles-cooper]

What I did

fix #3088, fix #2799

How I did it

How to verify it

Commit message

this commit fixes bad runtime encoding of unicode strings. `parse_Str`
used the utility function `string_to_bytes`, which rejects characters
with value larger than 255 and otherwise produces the ascii encoding
of the string. the issue is that bytes in the range 128-255 specify
different characters in utf-8 than in ascii encodings, resulting in
different values at runtime than at compile-time.

this can be seen from differing compile-vs-runtime behavior of
`keccak256` (this example was provided in GH issue 3088):

```vyper
@external @view def compile_hash() -> bytes32:
    return keccak256("è")

@external @view def runtime_hash() -> bytes32:
    s: String[1] = "è" return keccak256(s)
```

this commit fixes and simplifies `parse_Str` by using python's
`str.encode()` builtin, which encodes using utf-8 by default. it also
increases strictness of string validation to reject bytes in the range
128-255, since in utf-8 these can encode multibyte characters, which we
reject in vyper (see more discussion in GH issue 2338).

Description for the changelog

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.00%. Comparing base (9b5523e) to head (3b81ba0).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3091   +/-   ##
=======================================
  Coverage   91.99%   92.00%           
=======================================
  Files         119      119           
  Lines       16673    16662   -11     
  Branches     2803     2801    -2     
=======================================
- Hits        15339    15330    -9     
+ Misses        916      915    -1     
+ Partials      418      417    -1     

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[cyberthirst]

from what i understand the issue originated in how we encoded strings - different code sections used different encodings.

can we add an utility for encoding that would called whenever we need to encode something?

eg in keccak we now have

elif isinstance(value, vy_ast.Str):
            value = value.value.encode()

which relies on the default encoding - would be better to be explicit and specify the encoding and use the same function everywhere