Improve performance of partial backtraces

Previously, backtrace_each fully populated the rb_backtrace_t with all
backtrace frames, even if caller only requested a partial backtrace
(e.g. Kernel#caller_locations(1, 1)).  This changes backtrace_each to
only add the requested frames to the rb_backtrace_t.

To do this, backtrace_each needs to be passed the starting frame and
number of frames values passed to Kernel#caller or #caller_locations.

backtrace_each works from the top of the stack to the bottom, where the
bottom is the current frame.  Due to how the location for cfuncs is
tracked using the location of the previous iseq, we need to store an
extra frame for the previous iseq if we are limiting the backtrace and
final backtrace frame (the first one stored) would be a cfunc and not
an iseq.

To limit the amount of work in this case, while scanning until the start
of the requested backtrace, for each iseq, store the cfp.  If the first
backtrace frame we care about is a cfunc, use the stored cfp to find the
related iseq.  Use a function pointer to handle the storage of the cfp
in the iteration arg, and also store the location of the extra frame
in the iteration arg.

backtrace_each needs to return int instead of void in order to signal
when a starting frame larger than backtrace size is given, as caller
and caller_locations needs to return nil and not the empty array in
these cases.

To handle cases where a range is provided with a negative end, and the
backtrace size is needed to calculate the result to pass to
rb_range_beg_len, add a backtrace_size static function to calculate
the size, which copies the logic from backtrace_each.

As backtrace_each only adds the backtrace lines requested,
backtrace_to_*_ary can be simplified to always operate on the entire
backtrace.

Previously, caller_locations(1,1) was about 6.2 times slower for an
800 deep callstack compared to an empty callstack.  With this new
approach, it is only 1.3 times slower. It will always be somewhat
slower as it still needs to scan the cfps from the top of the stack
until it finds the first requested backtrace frame.

This initializes the backtrace memory to zero.  I do not think this is
necessary, as from my analysis, nothing during the setting of the
backtrace entries can cause a garbage collection, but it seems the
safest approach, and it's unlikely the performance decrease is
significant.

This removes the rb_backtrace_t backtrace_base member. backtrace
and backtrace_base were initialized to the same value, and neither
is modified, so it doesn't make sense to have two pointers.

This also removes LOCATION_TYPE_IFUNC from vm_backtrace.c, as
the value is never set.

Fixes [Bug #17031]

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