rehash: use associative array to hold registered shims

"hyperfine pyenv-rehash" before on my bash 4.4:

Time (mean ± σ):   172.8 ms ±   8.2 ms [User: 185.0 ms, System: 24.8 ms]
Range (min … max): 164.2 ms … 198.4 ms 15 runs

After:

Time (mean ± σ):   113.8 ms ±   2.8 ms [User: 127.1 ms, System: 26.1 ms]
Range (min … max): 108.0 ms … 117.6 ms 25 runs

libexec/pyenv-rehash

@@ -123,17 +123,19 @@ make_shims() {
   done
 }
 
-registered_shims=" "
+if ((${BASH_VERSINFO[0]} > 3)); then
+
+  declare -A registered_shims
 
   # Registers the name of a shim to be generated.
   register_shim() {
-  registered_shims="${registered_shims}${1} "
+    registered_shims["$1"]=1
   }
 
-# Install all the shims registered via `make_shims` or `register_shim` directly.
+  # Install all shims registered via `make_shims` or `register_shim` directly.
   install_registered_shims() {
     local shim file
-  for shim in $registered_shims; do
+    for shim in "${!registered_shims[@]}"; do
       file="${SHIM_PATH}/${shim}"
       [ -e "$file" ] || cp "$PROTOTYPE_SHIM_PATH" "$file"
     done
@@ -143,6 +145,31 @@ install_registered_shims() {
   # over the contents of the shims directory. Any file that is present
   # in the directory but has not been registered as a shim should be
   # removed.
+  remove_stale_shims() {
+    local shim
+    for shim in "$SHIM_PATH"/*; do
+      if [[ ! ${registered_shims["${shim##*/}"]} ]]; then
+        rm -f "$shim"
+      fi
+    done
+  }
+
+else # Same for bash < 4.
+
+    registered_shims=" "
+
+    register_shim() {
+      registered_shims="${registered_shims}${1} "
+    }
+
+    install_registered_shims() {
+      local shim file
+      for shim in $registered_shims; do
+        file="${SHIM_PATH}/${shim}"
+        [ -e "$file" ] || cp "$PROTOTYPE_SHIM_PATH" "$file"
+      done
+    }
+
     remove_stale_shims() {
       local shim
       for shim in "$SHIM_PATH"/*; do
@@ -151,6 +178,7 @@ remove_stale_shims() {
         fi
       done
     }
+fi
 
 shopt -s nullglob