Multiple adjustments to the Exception and raise logic.

[headius]

• Request backtrace from exception object before raise, so that
  pre-set backtrace or overridden #backtrace skip native trace
  gathering.
• Eliminate some redundant or unused backtrace-gathering methods.
• Fix Kernel#warn uplevel logic to use partial traces (Java 9).

The fixes here allow two additional ways of blunting the cost of
raising an exception (by eliminating the native stack trace):

• Call Exception#set_backtrace before raising
• Use an Exception subtype that overrides Exception#backtrace

This improves the performance of these scenarios to be roughly
equivalent to the three-arg form of Kernel#raise.

Fixes #5605.

[headius]

The three failures are the same three from master's -ea hang in MRI suite. I'm investigating that on master.