Call `rbenv-*` subcommands directly, not through the `rbenv` executable

Benefits:
- makes it easier to stub `rbenv-*` subcommands in tests
- speeds up subcommands because it skips the main `rbenv` executable

Caveats:
- users are no longer able to call `bin/rbenv-install` or
  `rbenv-uninstall` directly

bin/rbenv-install

@@ -94,7 +94,7 @@ unset VERSION_NAME
 # version is specified by rbenv. Show usage instructions if a local
 # version is not specified.
 DEFINITION="${ARGUMENTS[0]}"
-[ -n "$DEFINITION" ] || DEFINITION="$(rbenv local 2>/dev/null || true)"
+[ -n "$DEFINITION" ] || DEFINITION="$(rbenv-local 2>/dev/null || true)"
 [ -n "$DEFINITION" ] || usage 1
 
 
@@ -154,7 +154,7 @@ fi
 # REE installer requires an existing Ruby installation to run. An
 # unsatisfied local .ruby-version file can cause the installer to
 # fail.)
-export RBENV_VERSION="$(rbenv global 2>/dev/null || true)"
+export RBENV_VERSION="$(rbenv-global 2>/dev/null || true)"
 
 
 # Execute `before_install` hooks.
@@ -193,7 +193,7 @@ for hook in "${after_hooks[@]}"; do eval "$hook"; done
 
 # Run `rbenv-rehash` after a successful installation.
 if [ "$STATUS" == "0" ]; then
-  rbenv rehash
+  rbenv-rehash
 else
   cleanup
 fi

bin/rbenv-uninstall

@@ -76,7 +76,7 @@ for hook in "${before_hooks[@]}"; do eval "$hook"; done
 
 if [ -d "$PREFIX" ]; then
   rm -rf "$PREFIX"
-  rbenv rehash
+  rbenv-rehash
 fi
 
 for hook in "${after_hooks[@]}"; do eval "$hook"; done