to fix inconsistent and wrong current namespace detections.

This includes:
* Moving load_path and related things from rb_vm_t to rb_namespace_t to simplify
  accessing those values via namespace (instead of accessing either vm or ns)
* Initializing root_namespace earlier and consolidate builtin_namespace into root_namespace
* Adding VM_FRAME_FLAG_NS_REQUIRE for checkpoints to detect a namespace to load/require files
* Removing implicit refinements in the root namespace which was used to determine
  the namespace to be loaded (replaced by VM_FRAME_FLAG_NS_REQUIRE)
* Removing namespaces from rb_proc_t because its namespace can be identified by lexical context
* Starting to use ep[VM_ENV_DATA_INDEX_SPECVAL] to store the current namespace when
  the frame type is MAGIC_TOP or MAGIC_CLASS (block handlers don't exist in this case)

If there's a syntax error during iseq compilation then prism would leak
memory because it would not free the pm_parse_result_t.

This commit changes pm_iseq_new_with_opt to have a rb_protect to catch
when an error is raised, and return NULL and set error_state to a value
that can be raised by calling rb_jump_tag after memory has been freed.

For example:

    10.times do
      10_000.times do
        eval("/[/=~s")
      rescue SyntaxError
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    39280
    68736
    99232
    128864
    158896
    188208
    217344
    246304
    275376
    304592

After:

    12192
    13200
    14256
    14848
    16000
    16000
    16000
    16064
    17232
    17952

Refactor out the same code from `rb_builtin_ast_value` and
`pm_builtin_ast_value.

There is an English word "vast".
This commit changes the name to be more clear name to avoid confusion.