Add vm.assume metrics

[horsefacts]

Collect metrics on usage of vm.assume. Rather than using Foundry vm.assume, import { assume } from './VmUtils.sol' and use it just like vm.assume with an extra string argument:

assume(frobs > 0, "no_frobs");

"no_frobs" will be collected as a counter metric in assume-metrics.txt.

Run yarn test:fuzz:metrics assume-metrics.txt to generate a visualization.

Also:

• Extract metrics loggers to Metrics.sol
• Fixes some compiler warnings (unused args, mutability)

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3caa72c) 100.00% compared to head (832df19) 100.00%.

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Additional details and impacted files

@@            Coverage Diff            @@
##               1.5     #1154   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           46        46           
  Lines         2652      2652           
  Branches       396       396           
=========================================
  Hits          2652      2652           

Flag	Coverage Δ
foundry	77.80% <ø> (ø)
production	100.00% <ø> (ø)
reference	100.00% <ø> (ø)

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