xpath like expressions for test verification
xpath like expression with index
xpath like expression walker moved to own class
Merge branch 'xpath'
formalParameter may be empty parentheses
parameter type introduced to cope with qualId

(type for classes)
function may return string
allow return type and paremeter default of qualid
allow parameter type array of string
fail on messages to stderr

actually it is possible to make antlr4 to throw an exception in case of
failure; but to do so it is necessary to change the grammar (it is not a
runtime parameter); the parsing recovery is actually a good feature, so
catching messages going to stderr is a good compromise of a resilient
parser and strict testing.
restructuring tests to avoid the free ride antipattern
implementation section uses clause
implementation const section