Refactor generate functions to a separate file

[cjbarth]

In the spirit of #19, move some functions out of the very long saml.ts to another file without breaking class semantics. This will make it easier to land PRs with less chance of conflicts going forward.

[codecov]

Codecov Report

Merging #129 (4582e8e) into master (4355f8f) will increase coverage by 0.31%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
+ Coverage   80.19%   80.51%   +0.31%     
==========================================
  Files          11       12       +1     
  Lines         808      821      +13     
  Branches      243      243              
==========================================
+ Hits          648      661      +13     
  Misses         72       72              
  Partials       88       88              

Impacted Files	Coverage Δ
src/saml/generate.ts	67.74% <67.74%> (ø)
src/metadata.ts	85.00% <100.00%> (ø)
src/saml.ts	80.69% <100.00%> (+2.69%)	⬆️

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[markstos]

If I'm reading this correctly, one key difference with #19, is that here the smaller file that's broken out still retains a this that refers to a SAML object, while #19 took a more aggressive approach by creating functions that have explicit inputs and outputs.

So while #19 prioritizes code that is easier to reason about, this PR takes a compromise approach of splitting up files which might help avoid conflicts. This PR also retains more internal API compatibility.

I'm going to follow-up more in #131.