Adds doc comments, Element::write_all_as method

[zslayton]

• Adds doc comments to the Int type.
• Renames the Element::integer(...) constructor to Element::int(...) for consistency with the IonType::* names.
• Adds an Element::write_all_as that works with any iterable collection of Element. This method allows a stream with more than one top-level value to be written, resulting in more compact output through symtab reuse.

Partially addresses #587.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Patch coverage: 11.11% and project coverage change: -0.07 ⚠️

Comparison is base (202bd2b) 82.45% compared to head (74154ef) 82.39%.

❗ Current head 74154ef differs from pull request most recent head 4f7b370. Consider uploading reports for the commit 4f7b370 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #604      +/-   ##
==========================================
- Coverage   82.45%   82.39%   -0.07%     
==========================================
  Files         111      111              
  Lines       20032    20048      +16     
  Branches    20032    20048      +16     
==========================================
  Hits        16518    16518              
- Misses       1896     1912      +16     
  Partials     1618     1618              

Impacted Files	Coverage Δ
src/element/builders.rs	96.84% <ø> (ø)
src/element/sequence.rs	98.18% <ø> (ø)
src/lib.rs	0.00% <ø> (ø)
src/types/integer.rs	86.71% <ø> (ø)
src/types/list.rs	67.50% <ø> (ø)
src/types/sexp.rs	60.00% <ø> (ø)
src/element/mod.rs	80.36% <5.88%> (-1.52%)	⬇️
src/types/struct.rs	88.76% <100.00%> (ø)

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