Experimental: simplify data structs

[alovak]

The goal of this PR is to implement (now it's more like proof of concept) the proposal #277 to use go basic types in the data structs instead of *field.String, *field.Numeric, etc. like this:

	type authRequestData struct {
		MTI                  string `index:"0"`
		PrimaryAccountNumber string `index:"2"`
		ProcessingCode       int    `index:"3"`
	}

[codecov-commenter]

Codecov Report

Patch coverage: 46.07% and project coverage change: -1.10% ⚠️

Comparison is base (b652df4) 73.70% compared to head (3d54372) 72.60%.

❗ Current head 3d54372 differs from pull request most recent head 255001f. Consider uploading reports for the commit 255001f to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
- Coverage   73.70%   72.60%   -1.10%     
==========================================
  Files          43       44       +1     
  Lines        2282     2351      +69     
==========================================
+ Hits         1682     1707      +25     
- Misses        369      412      +43     
- Partials      231      232       +1     

Files Changed	Coverage Δ
field/numeric.go	65.13% <10.00%> (-18.01%)	⬇️
field/string.go	70.96% <25.00%> (-12.17%)	⬇️
message.go	72.85% <70.00%> (+0.90%)	⬆️
field_tag.go	72.97% <72.97%> (ø)
field/composite.go	82.01% <100.00%> (+0.13%)	⬆️

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[alovak]

this PR was a proof of concept. Now all ideas are implemented in the module.