Bring automatic gem-rehash functionality to rbenv core

This bakes in the functionality of rbenv-gem-rehash plugin.

The Rubygems hook is improved:

- It will not rehash for gems installed in locations that rbenv otherwise
  doesn't search for binstubs; for instance in case of
  `bundle --path vendor/bundle`.
- It rescues exceptions and makes them non-lethal by warning on stderr.

rbenv.d/exec/gem-rehash.bash

@@ -0,0 +1 @@
+export RUBYLIB="${BASH_SOURCE%.bash}:$RUBYLIB"

rbenv.d/exec/gem-rehash/rubygems_plugin.rb

@@ -0,0 +1,18 @@
+hook = lambda do |installer|
+  begin
+    # Ignore gems that aren't installed in locations that rbenv searches for binstubs
+    if installer.spec.executables.any? &&
+        [Gem.default_bindir, Gem.bindir(Gem.user_dir)].include?(installer.bin_dir)
+      system "rbenv", "rehash"
+    end
+  rescue
+    warn "rbenv: error in gem-rehash (#{$!})"
+  end
+end
+
+begin
+  Gem.post_install(&hook)
+  Gem.post_uninstall(&hook)
+rescue
+  warn "rbenv: error installing gem-rehash hooks (#{$!})"
+end