- iseq.c: add following two methods.
- ISeq#line_trace_all returns all line traces (line numbers)
- ISeq#line_trace_specify(pos, set) set `pos'th line event to specified_line event (if set is true). These features are introduced for debuggers (mainly to make breakpoint).
- iseq.h: add decl. of C APIs.
- test/ruby/test_iseq.rb: add tests.
- vm_trace.c: add `specified_line' event.
- include/ruby/ruby.h: ditto.