Removed implementation of Readline.point

This was included in rb-readline in commit a489195d57.

Tested-by: Mark Somerville <mark@scottishclimbs.com> [Arch, MRI 2.0.0]
Signed-off-by: Mark Somerville <mark@scottishclimbs.com>
Add _ builtin to manipulate running Urchin instance

Tested-by: Mark Somerville <mark@scottishclimbs.com> [Arch, MRI 2.0.0]
Signed-off-by: Mark Somerville <mark@scottishclimbs.com>