Speed up rebuilding the loaded feature index

	Rebuilding the loaded feature index slowed down with the bug fix
	for #17885 in 79a4484a.  The
	slowdown was extreme if realpath emulation was used, but even when
	not emulated, it could be about 10x slower.

	This adds loaded_features_realpath_map to rb_vm_struct. This is a
	hidden hash mapping loaded feature paths to realpaths. When
	rebuilding the loaded feature index, look at this hash to get
	cached realpath values, and skip calling rb_check_realpath if a
	cached value is found.

	Fixes [Bug #19246]
	---
	 load.c    | 27 +++++++++++++++++++++++----
	 vm.c      |  2 ++
	 vm_core.h |  1 +
	 3 files changed, 26 insertions(+), 4 deletions(-)