Reduce public interface by making some methods private

[cjbarth]

Every method was public, but wasn't designed to be. Methods that weren't used in tests are marked as private in this PR.

[codecov]

Codecov Report

Merging #394 (9f68078) into master (b0f00d0) will increase coverage by 1.47%.
Report is 1 commits behind head on master.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
+ Coverage   72.98%   74.46%   +1.47%     
==========================================
  Files           9        9              
  Lines         881      881              
  Branches      235      234       -1     
==========================================
+ Hits          643      656      +13     
+ Misses        149      132      -17     
- Partials       89       93       +4     

Files	Coverage Δ
src/types.ts	90.00% <ø> (ø)
src/utils.ts	85.18% <100.00%> (+0.13%)	⬆️
src/enveloped-signature.ts	82.60% <50.00%> (+0.79%)	⬆️
src/exclusive-canonicalization.ts	77.34% <50.00%> (ø)
src/c14n-canonicalization.ts	52.10% <25.00%> (ø)
src/signed-xml.ts	77.37% <54.54%> (+3.03%)	⬆️

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