Enables explicit API mode

[popematt]

Issue #, if available:

Fixes #47

Description of changes:

Enables Explicit API mode. This requires us to be explicit about what is part of the API, and to be explicit about the return types of any public functions.

In most cases, it was obvious that the class/interface/val/fun should be public, but there are a few cases that I was not completely sure about.

• typealias Annotations -- appears to only be used internally, but another typealias (MetaContainer) does already have confirmed external usage.
• IonElementException constructors -- while the exceptions themselves should be public, should the constructors be public?

In both cases, I have chosen to be conservative and label them as internal, but I'm definitely open to be convinced otherwise.

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

[codecov-commenter]

Codecov Report

Merging #71 (44f0c5f) into master (a56988f) will decrease coverage by 0.01%.
The diff coverage is 68.57%.

@@             Coverage Diff              @@
##             master      #71      +/-   ##
============================================
- Coverage     88.07%   88.06%   -0.02%     
  Complexity      453      453              
============================================
  Files            30       30              
  Lines           822      821       -1     
  Branches         72       72              
============================================
- Hits            724      723       -1     
  Misses           65       65              
  Partials         33       33              

Impacted Files	Coverage Δ
src/com/amazon/ionelement/api/IonUtils.kt	100.00% <ø> (ø)
src/com/amazon/ionelement/api/ElementType.kt	87.23% <28.57%> (ø)
.../com/amazon/ionelement/api/IonElementExtensions.kt	33.33% <33.33%> (ø)
...c/com/amazon/ionelement/api/IonElementException.kt	50.00% <50.00%> (ø)
src/com/amazon/ionelement/api/IonLocation.kt	66.66% <66.66%> (-4.77%)	⬇️
src/com/amazon/ionelement/api/Ion.kt	98.27% <100.00%> (ø)
src/com/amazon/ionelement/api/IonElement.kt	100.00% <100.00%> (ø)
src/com/amazon/ionelement/api/IonElementLoader.kt	100.00% <100.00%> (ø)
src/com/amazon/ionelement/api/IonMeta.kt	62.50% <100.00%> (ø)
...rc/com/amazon/ionelement/impl/ByteArrayViewImpl.kt	40.00% <100.00%> (ø)
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a56988f...44f0c5f. Read the comment docs.