Misc tidying up around KclValue, etc. (#5653)

* Make some things a little less public

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* Rename KclValue::Array to MixedArray

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* Add settings to KclValue::Function

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* Remove a bunch of dead code, simplifying mock exec and meaning KclValue no longer needs to deserialize

Signed-off-by: Nick Cameron <nrc@ncameron.org>

---------

Signed-off-by: Nick Cameron <nrc@ncameron.org>

rust/kcl-lib/tests/array_elem_push/program_memory.snap

@@ -1,10 +1,10 @@
 ---
-source: kcl/src/simulation_tests.rs
+source: kcl-lib/src/simulation_tests.rs
 description: Variables in memory after executing array_elem_push.kcl
 ---
 {
   "arr": {
-    "type": "Array",
+    "type": "MixedArray",
     "value": [
       {
         "type": "Number",
@@ -84,7 +84,7 @@ description: Variables in memory after executing array_elem_push.kcl
     ]
   },
   "new_arr1": {
-    "type": "Array",
+    "type": "MixedArray",
     "value": [
       {
         "type": "Number",
@@ -186,7 +186,7 @@ description: Variables in memory after executing array_elem_push.kcl
     ]
   },
   "new_arr2": {
-    "type": "Array",
+    "type": "MixedArray",
     "value": [
       {
         "type": "Number",