cannon: Modularize mipsevm packages

[mbaxter]

Description

Refactor the mipsevm package to be more modular.

Basically, this PR creates a bunch of sub-packages for the various files that were previously in the mipsevm package. Generically, this is meant to help create a better separation of concerns, clarify dependencies, improve encapsulation, and make test utilities more reusable. It should also help keep the code legible as we add the new Multithreaded Cannon logic and avoid name conflicts and accidental imports.

Overview of the new package structure:

• A new test_util package that holds test helpers extracted from the existing tests
• A tests package to hold non-unit tests
  • This just helps remove some noise and make it clearer which files are production files
• Move implementation-specific logic (original vs MT Cannon) to subpackages

Other changes:

• Rename MTState to State (and similarly for related constants, variables, etc)
• Replace some implementation-specific parameters (State) with the generic interface (FPVMState).

Notes for the reviewer:
There should be no functional changes in this PR. The changes here should mostly consist of package restructuring, visibility changes to some symbols, and shuffling some symbols between files.

Tests

This is a refactor, tests were just moved. One thing to highlight is that some tests in state_test.go were moved to instrumented_test.go to avoid circular dependencies (and because these tests were not really for state.go).

Metadata

Part of: #11642

[codecov]

Codecov Report

Attention: Patch coverage is 79.38596% with 94 lines in your changes missing coverage. Please review.

Project coverage is 64.41%. Comparing base (f5221f4) to head (15e41f5).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #11135      +/-   ##
===========================================
+ Coverage    62.58%   64.41%   +1.83%     
===========================================
  Files           21       27       +6     
  Lines         1935     2147     +212     
  Branches        71       71              
===========================================
+ Hits          1211     1383     +172     
- Misses         687      717      +30     
- Partials        37       47      +10     

Flag	Coverage Δ
cannon-go-tests	80.33% <79.38%> (+0.12%)	⬆️
chain-mon-tests	27.14% <ø> (ø)
sdk-tests	16.44% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
cannon/mipsevm/exec/mips_instructions.go	97.84% <100.00%> (ø)
cannon/mipsevm/exec/mips_syscalls.go	93.33% <100.00%> (ø)
cannon/mipsevm/memory/memory.go	82.89% <ø> (ø)
cannon/mipsevm/memory/page.go	66.21% <ø> (ø)
cannon/mipsevm/program/metadata.go	0.00% <ø> (ø)
cannon/mipsevm/singlethreaded/mips.go	69.52% <100.00%> (ø)
cannon/mipsevm/state.go	100.00% <100.00%> (+12.72%)	⬆️
cannon/mipsevm/testutil/elf.go	100.00% <100.00%> (ø)
cannon/mipsevm/witness.go	100.00% <ø> (ø)
cannon/mipsevm/program/patch.go	75.51% <75.00%> (ø)
... and 8 more

[Inphi]

Consider flattening Go packages unless there are visibility concerns.
Specifically, the following files should be directly part of the core package rather than in subpackages:

• core/witness/witness.go
• core/oracle/oracle.go
• core/state/state.go
• core/debug/debug.go

This frees us identifier uses for debug, state, etc without introducing ambiguity.

Similarly, the impls package doesn't serve any purpose. We can have single_threaded and multi_threaded packages moved up.

[mbaxter]

Consider flattening Go packages unless there are visibility concerns.

Flattened those packages as you suggested, and went ahead and moved the core packages back out to the root mipsevm after reading through that article you posted. Still trying to wrap my head around idiomatic go and I think I went a little overboard once it kind of clicked that encapsulation only happens at the package level :D

[Inphi]

Nice!

[Inphi]

Need a monorepo-ops-reviewer for approval. cc @ajsutton