Separate application-defined mapping logic from the Saga itself.

[jmalloc]

Fixes #111

There was no need that these methods be on the same value as the saga.Saga implementation. Decoupling them in this way allows for more flexible configuration, including the two new ByField() convenience functions:

• direct.ByField() returns a direct.Mapper that determines the saga instance ID by concatenating 1 or more fields from the message
• keyset.ByField() returns a keyset.Mapper that determines a mapping key by concatenating 1 or more fields from the message, and correlating with fields of the same name in the saga instance. Instance IDs are randomly generated UUIDs

[codecov-io]

Codecov Report

Merging #112 into master will decrease coverage by 0.1%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   30.17%   30.07%   -0.11%     
==========================================
  Files          65       65              
  Lines        2260     2268       +8     
==========================================
  Hits          682      682              
- Misses       1567     1575       +8     
  Partials       11       11

Impacted Files	Coverage Δ
src/ax/saga/mapping/direct/mapper.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/mapping/keyset/byfield.go	0% <0%> (ø)
src/ax/saga/mapping/keyset/mapper.go	0% <0%> (ø)	:arrow_up:
src/ax/aggregate/saga.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/mapping/direct/byfield.go	0% <0%> (ø)

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