path_parser.rs: Make PathParser private, and only have a public parse_path_into_builder() function
path_parser.rs: Make PathParser private, and only have a public parse_path_into_builder() function

This function returns a Result<(), ParseError>, where ParseError is a
new struct.

Eventually we'll have tests for actual error reporting.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg