Refactor out all mssql defined data types into a mssqltypes subpackage

[yukiwongky]

Part of issue #357

[codecov[bot]]

Codecov Report

Merging #518 into master will decrease coverage by 0.17%. The diff coverage is 26.31%.

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
- Coverage   68.67%   68.49%   -0.18%     
==========================================
  Files          22       20       -2     
  Lines        5056     4984      -72     
==========================================
- Hits         3472     3414      -58     
+ Misses       1379     1367      -12     
+ Partials      205      203       -2

Impacted Files	Coverage Δ
mssql.go	81.59% <0%> (ø)	:arrow_up:
mssql_go19.go	95.65% <100%> (ø)	:arrow_up:
buf.go	100% <100%> (ø)	:arrow_up:
types.go	77.29% <16.66%> (ø)	:arrow_up:
bulkcopy.go	57.74% <44.44%> (ø)	:arrow_up:
token.go	55.91% <7.54%> (ø)	:arrow_up:

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[codecov[bot]]

Codecov Report

Merging #518 into master will decrease coverage by 0.13%. The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master     #518      +/-   ##
==========================================
- Coverage   68.86%   68.72%   -0.14%     
==========================================
  Files          22       20       -2     
  Lines        5058     5004      -54     
==========================================
- Hits         3483     3439      -44     
+ Misses       1369     1361       -8     
+ Partials      206      204       -2

Impacted Files	Coverage Δ
error.go	75% <ø> (ø)	:arrow_up:
mssql_go19.go	95.65% <100%> (ø)	:arrow_up:
types.go	77.29% <100%> (ø)	:arrow_up:
bulkcopy.go	57.74% <44.44%> (ø)	:arrow_up:
mssql_go19pre.go

---

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[yukiwongky]

I've pulled Error out from this PR. @denisenkom can you check?

[denisenkom]

This needs to be rebased