refactor: make partial kernels possible

[Stebalien]

This change has two parts:

1. Remove most (all except gas) "ops" from the base kernel trait, allowing kernels to implement only some operations.
2. Make syscalls declare the ops they actually need instead of depending on the entire kernel.

This should make it possible to implement custom kernels that expose fewer operations.

It also removes the LimiterOps trait as that's not really an "op".

[codecov-commenter]

Codecov Report

Merging #1941 (3d5571d) into master (6867921) will decrease coverage by 0.02%.
The diff coverage is 80.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1941      +/-   ##
==========================================
- Coverage   75.70%   75.68%   -0.02%     
==========================================
  Files         157      157              
  Lines       15473    15483      +10     
==========================================
+ Hits        11714    11719       +5     
- Misses       3759     3764       +5     

Files	Coverage Δ
fvm/src/kernel/filecoin.rs	22.65% <100.00%> (+0.60%)	⬆️
fvm/src/kernel/mod.rs	90.00% <ø> (-0.91%)	⬇️
fvm/src/syscalls/crypto.rs	100.00% <100.00%> (ø)
fvm/src/syscalls/event.rs	95.83% <100.00%> (ø)
fvm/src/syscalls/ipld.rs	100.00% <100.00%> (ø)
fvm/src/syscalls/mod.rs	97.39% <ø> (+0.52%)	⬆️
fvm/src/syscalls/network.rs	42.85% <100.00%> (ø)
fvm/src/syscalls/sself.rs	100.00% <100.00%> (ø)
fvm/src/syscalls/vm.rs	62.16% <100.00%> (+2.16%)	⬆️
fvm/src/syscalls/actor.rs	89.91% <85.71%> (ø)
... and 4 more

... and 5 files with indirect coverage changes

[maciejwitowski]

These changes are all compiler-checked so I'm happy to approve them.