Update VM trait

[alexytsu]

• update VM trait to align with anorth/fvm-workbench

Some methods that were previously on the abstract VM are deemed specific to the implementation of the TestVM in builtin-actors. To ease alignment with VM implementations in other contexts such as https://github.com/anorth/fvm-workbench, certain methods have been removed from the trait such as those relating to builtin-actors' notion of Policy and actor manifest.

[codecov-commenter]

Codecov Report

Merging #1340 (76c83cf) into master (a77674d) will decrease coverage by 0.06%.
The diff coverage is 73.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
- Coverage   90.84%   90.79%   -0.06%     
==========================================
  Files         138      139       +1     
  Lines       27350    27351       +1     
==========================================
- Hits        24847    24832      -15     
- Misses       2503     2519      +16     

Files Changed	Coverage Δ
test_vm/src/vm.rs	43.47% <43.47%> (ø)
test_vm/src/lib.rs	82.68% <82.35%> (+0.35%)	⬆️
state/src/check.rs	86.08% <100.00%> (-0.27%)	⬇️
test_vm/src/util/mod.rs	99.42% <100.00%> (ø)

... and 4 files with indirect coverage changes

[anorth]

Looks good. There's some deconflicting to do with #1343 and follow up to cut clean deps, but that looks easy enough from here.