ExpressionType: modify to mimic suggested RFC algorithm
VariableValue: add .isEmpty()
ExpressionType: make all members package local
URITemplateExpression: add .expand2() method plus stubs
VariableSpec: add .isExploded(), .getPrefixLength()

Prefix length is -1 for non prefixed variables
Implement .expand2() for TemplateLiteral
New class CharMatchers

For now, it contains the reserved and unreserved matchers used for string
expansion.
Factorize reading variable values from JSON

Could do with being integrated in main sources eventually. But URI templates are
not always used in conjuction with JSON.
Start to test new algorithm
Change test file format
Add more simple tests
More simple strings tests
Complete tests for single strings
Add multiple strings expansion tests
Tentative, non tested code for maps/lists expansion
Start and add list expansion tests
Add expansion tests for empty list
Add further list expansion tests
More lists expansion examples
ValueType: remove UNDEFINED

It was not used.