Sanitised identifiers + all_of/any_of support

Sanitise identifiers, better all_of / any_of support, and a large refactoring

• Sanitises / Elmifies JSON schema identifiers, such that the Elm code output is valid Elm

• Moves indentation- and naming-specific logic from printer/util.ex into a new
  printer/utils folder,
• adjusts all printers to now use the Naming.normalize_identifier() function
  before sending identifier names to Elm code templates, and
• adds tests for the 'sanitise identifier' feature.

• Fixes and refactors the allOf and anyOf printers, such that they produce correct Elm code for decoding and encoding all_of and any_of JSON schema nodes.

• Adds contexts 'parser' and 'printer'

We isolate all parser related code in the parser folder and expose the
parser.ex module as the interface for that folder / context.

We repeat this exercise for the printer folder / context.

Furthermore, we remove all the import statements and replace them with more
explicit aliases.

• Splits the JS2E.Printer.Util module into a whole printer/utils
  folder in order to increase cohesion, i.e. have one util module per relevant
  area of printing.

• Fixes all dialyzer errors except for allOf/anyOf/oneOf printers

• Improves documentation

• Updates 'allOf' and 'anyOf' type descriptions to reflect new Elm
  decoders/encoders,
• updates README.md to include a section on js2e error reporting, and
• creates a CONTRIBUTING.md file, detailing what potential contributors should
  know before filing issues/PRs.