Accept a colon in the variable name of a `<<...>>` call and of an
`((...))` attribute reference, restoring the v5.3.8 name character
class. Call Syntax already describes the callee name as any
non-whitespace character other than `(` or `>`, and the colon is only
meaningful at the parameter name position.
* Add test cases for a colon name standalone, as an attribute value, as
an =-separated parameter value, and with named parameters
* Add the 5.5.0 change note
Mario Pietsch
5577a3d07b