deprecation process for a nice api design (as discussed in #124)

deprecate:

- definition
- goto
- complete
- related_names
- function_definition

in favor of:

- goto_definitions
- goto_assignments
- completions
- usages
- call_signatures

Thank you @dbrgn and @tkf for the fruitful discussion!
Script.complete has been deprecated, therefore changed all usages / documatation to Script.completions
deprecated api_classes.Completion.word in favor of name
make some variables of the Completion class private
more variables private in the same file
make api_classes.BaseDefinition._definition private
removed related_names occurences in favor of usages
api_classes.RelatedName -> Usage
removed a lot of the goto usages, used goto_assignments instead. Did the same also for a lot of definition uses
change some definition usages to goto_definitions in tests and docs
removed an old part_parser test