Add convenience functions for parsing/generating various IDs

[koden-km]

Fixes #45

[koden-km]

I wasn't sure if i should change all places doing MessageID generation. I left the places currently doing it after envelope creation, like this:

env.MessageID.GenerateUUID()

[codecov-io]

Codecov Report

Merging #121 into master will increase coverage by 0.78%. The diff coverage is 93.93%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   29.27%   30.05%   +0.78%     
==========================================
  Files          66       67       +1     
  Lines        2330     2352      +22     
==========================================
+ Hits          682      707      +25     
+ Misses       1637     1634       -3     
  Partials       11       11

Impacted Files	Coverage Δ
src/ax/saga/mapping/direct/byfield.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/mapping/keyset/byfield.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/instance.go	100% <100%> (ø)
src/ax/envelope.go	100% <100%> (ø)	:arrow_up:
src/ax/message.go	100% <100%> (ø)	:arrow_up:

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[jmalloc]

I wasn't sure if i should change all places doing MessageID generation

Yeah, anywhere that can be changed would be great. I only did them that way because I didn't have these sweet functions.

[koden-km]

I would need to replace the values in the envelope though, not just mutate the existing ones. Is that safe?

[koden-km]

Ah i guess i just use the new functions inside the construction braces. I'm still getting used to Go.