Traits and impls for adding context and exit codes to results.

[anorth]

This is a backport from the decouple-fil+ branch. The new traits and conversion methods are extracted from @dignifiedquire's work in #214. We can have them now without necessarily following through all the work to convert everything over. I've converted a couple of the simpler actors to demonstrate, and we should use the new traits for new code.

[anorth]

@Kubuxu you had questions about this in another PR. I believe these methods are still cheap because in implementation they still defer work through a closure. But with nice clean syntax. The with_ form should be used whenever a format!() is to be done.

[Kubuxu]

100% agreed that the pattern is nicer, I just noticed that the closure pattern wasn't used in many places where format! was used.

[Stebalien]

I have to say this is a lot nicer.

[Kubuxu]

@Stebalien is there any simple way for us to get a lint for cases like ones I've pointed out above?
I think it is similar to this clippy lint: https://rust-lang.github.io/rust-clippy/master/#or_fun_call but I have no idea if we can somehow tack it onto our functions.

---

In 2min of searching I found this: https://github.com/trailofbits/dylint but introducing it would be quite a hassle just for this.

[Stebalien]

is there any simple way for us to get a lint for cases like ones I've pointed out above?

I've never seen one, unfortunately.

[codecov-commenter]

Codecov Report

Merging #589 (847b390) into master (bacf044) will increase coverage by 0.55%.
The diff coverage is 81.54%.

❗ Current head 847b390 differs from pull request most recent head b835a0f. Consider uploading reports for the commit b835a0f to get more accurate results

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
+ Coverage   83.92%   84.48%   +0.55%     
==========================================
  Files          88       88              
  Lines       18155    18081      -74     
==========================================
+ Hits        15237    15275      +38     
+ Misses       2918     2806     -112     

Impacted Files	Coverage Δ
runtime/src/actor_error.rs	61.38% <58.82%> (-2.62%)	⬇️
actors/multisig/src/state.rs	94.79% <75.00%> (-4.05%)	⬇️
actors/multisig/src/lib.rs	95.23% <91.66%> (+15.50%)	⬆️
actors/init/src/lib.rs	96.77% <100.00%> (+1.38%)	⬆️
actors/init/src/state.rs	100.00% <100.00%> (ø)
actors/paych/src/lib.rs	92.71% <100.00%> (-0.15%)	⬇️
actors/system/src/lib.rs	94.23% <100.00%> (+1.63%)	⬆️
actors/verifreg/src/lib.rs	75.77% <100.00%> (+2.91%)	⬆️
runtime/src/builtin/shared.rs	92.30% <100.00%> (-1.45%)	⬇️
state/src/check.rs	87.23% <0.00%> (-0.43%)	⬇️
... and 7 more