Consider activation frame for stack height metering (#2)

* Charge a base cost for calling a function

* Added regression test for empty functions

* Satisfy clippy

tests/expectations/stack-height/imports.wat

@@ -13,7 +13,7 @@
     local.get 0
     local.get 1
     global.get 0
-    i32.const 2
+    i32.const 4
     i32.add
     global.set 0
     global.get 0
@@ -24,7 +24,7 @@
     end
     call 2
     global.get 0
-    i32.const 2
+    i32.const 4
     i32.sub
     global.set 0)
   (global (;0;) (mut i32) (i32.const 0))