refactor: #parse_value(): use table instead of case-when
refactor: move constants to just before methods which refers them
Add Context#succ for deprecate #next in future.
Remove needless `self.'
Add Context#parse and change access attr of other parse methods
remove: module PJSON
rename: class PJSON::Context -> PJSON
refactor: #parse_number use table
add #parse_unicode() to devide unicode part from string escape parser
refactor: use table to parse string escape sequences