tune code size / gas for bytes copy routines

[charles-cooper]

What I did

fix #2656, also unroll copy_bytes when the number of words is small (<8).

should refactor a little because a couple code paths look suspiciously similar. also consider exposing the constants UNROLL_WORD_COPY_TUNING and ROLL_ARRAY_TUNING as optimizer options.

How I did it

How to verify it

check IR following contract -- foo has loops, bar has unrolled, baz has loops

w: uint256[15]
@external
def foo(x: uint256[15]) -> uint256[15]:
    y: uint256[15] = x
    z: uint256[15] = y
    self.w = z
    return z

w2: uint256[5]
@external
def bar(x: uint256[5]) -> uint256[5]:
    y: uint256[5] = x
    z: uint256[5] = y
    self.w2 = z
    return z

@external
def baz(x: int128[7]) -> int128[7]:
    return x

Commit message

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Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #2738 (591ba5c) into master (6fdeb84) will decrease coverage by 18.29%.
The diff coverage is 60.39%.

@@             Coverage Diff             @@
##           master    #2738       +/-   ##
===========================================
- Coverage   86.66%   68.37%   -18.30%     
===========================================
  Files          93       93               
  Lines        9886     9910       +24     
  Branches     2420     2423        +3     
===========================================
- Hits         8568     6776     -1792     
- Misses        831     2458     +1627     
- Partials      487      676      +189     

Impacted Files	Coverage Δ
vyper/codegen/ir_node.py	90.60% <ø> (-1.88%)	⬇️
vyper/codegen/stmt.py	67.46% <0.00%> (-21.43%)	⬇️
vyper/codegen/core.py	67.04% <61.00%> (-17.05%)	⬇️
vyper/ast/natspec.py	12.34% <0.00%> (-86.42%)	⬇️
vyper/ir/s_expressions.py	5.88% <0.00%> (-85.30%)	⬇️
vyper/cli/utils.py	16.66% <0.00%> (-71.43%)	⬇️
vyper/codegen/function_definitions/utils.py	28.57% <0.00%> (-71.43%)	⬇️
vyper/cli/vyper_json.py	9.38% <0.00%> (-70.40%)	⬇️
vyper/compiler/output.py	29.37% <0.00%> (-64.41%)	⬇️
vyper/compiler/utils.py	36.00% <0.00%> (-56.00%)	⬇️
... and 51 more

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[charles-cooper]

superseded by #3483