refactor[codegen]: refactor assembly labels

[charles-cooper]

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Commit message

this commit refactors label handling in assembly. previous, how symbols
were handled was context dependent -- `_sym_` items followed by a
`JUMPDEST` were added to the symbol map, and `_sym_` items not followed
by a `JUMPDEST` would compile to `PUSH <symbol>`. this commit refactors
the structure to be more explicit, so that there is a clear distinction
between `PUSHLABEL` and `LABEL`. a new `PUSH_OFST` directive has
been added. `_mem_` ("memory symbols") which require a different PUSH
length, have not been handled in this refactor.

this commit also removes explicit jumpdests from the assembly format,
since `LABEL`s will now compile to `JUMPDEST`.

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[codecov]

Codecov Report

Attention: Patch coverage is 94.90446% with 8 lines in your changes missing coverage. Please review.

Project coverage is 92.50%. Comparing base (6b670e7) to head (42a4436).

Files with missing lines	Patch %	Lines
vyper/ir/compile_ir.py	95.65%	4 Missing and 2 partials ⚠️
vyper/venom/venom_to_assembly.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4629      +/-   ##
==========================================
- Coverage   92.51%   92.50%   -0.01%     
==========================================
  Files         130      130              
  Lines       18586    18637      +51     
  Branches     3225     3230       +5     
==========================================
+ Hits        17195    17241      +46     
- Misses        945      948       +3     
- Partials      446      448       +2     

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