[4.0] Lower case enums and better Swift 4 compatibility

[norio-nomura]

• Change cases of IndexingError to lowercase
  • Init(instance: AnyObject) is renamed to initialize(instance: AnyObject) since init is recognized as initializer.
• Change cases of XMLIndexer to lowercase
• @available of XMLIndexer.Element is guarded by #if !swift(>=3.2) for avoiding SR-4951.
  Current Swift 4 compiler with -swift-version 3 seems Swift 3.2. It may be necessary to change this when Xcode 9 beta starts in the future.

[codecov-io]

Codecov Report

Merging #134 into version-4.0-changes will decrease coverage by 0.43%.
The diff coverage is 37.71%.

@@                  Coverage Diff                   @@
##           version-4.0-changes    #134      +/-   ##
======================================================
- Coverage                27.63%   27.2%   -0.44%     
======================================================
  Files                       13      13              
  Lines                     1885    1915      +30     
======================================================
  Hits                       521     521              
- Misses                    1364    1394      +30

Impacted Files	Coverage Δ
Tests/SWXMLHashTests/XMLParsingTests.swift	0% <0%> (ø)	⬆️
Source/SWXMLHash.swift	76.9% <35.13%> (-5.38%)	⬇️
Source/SWXMLHash+TypeConversion.swift	61.99% <43.58%> (ø)	⬆️

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