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/fixtures/stack-height/empty_functions.wat

@@ -0,0 +1,8 @@
+(module
+  (func (;0;)
+    call 0
+  )
+  (func (;1;) (export "main")
+    call 0
+  )
+)

tests/fixtures/stack-height/global.wat

@@ -6,8 +6,8 @@
 
   (func $i32.add (export "i32.add") (param i32 i32) (result i32)
     get_local 0
-	get_local 1
-	i32.add
+	  get_local 1
+	  i32.add
   )
   (func (param $arg i32)
      (local $tmp i32)
@@ -15,7 +15,7 @@
      global.get 0
      i32.const 1
      i32.add
-	 tee_local $tmp
+	   tee_local $tmp
      global.set $counter
 
      get_local $tmp

tests/fixtures/stack-height/start.wat

@@ -3,7 +3,7 @@
 	(import "env" "memory" (memory 1 1))
 
 	(start $start)
-	(func $start (export "exported_start")
+	(func $start
 		(local i32)
 	)
 	(func (export "call")

tests/fixtures/stack-height/table.wat

@@ -1,15 +1,15 @@
 (module
   (import "env" "foo" (func $foo))
   (func (param i32)
-     get_local 0
-     i32.const 0
-     call $i32.add
-     drop
+    get_local 0
+    i32.const 0
+    call $i32.add
+    drop
   )
   (func $i32.add (export "i32.add") (param i32 i32) (result i32)
     get_local 0
-	get_local 1
-	i32.add
+	  get_local 1
+	  i32.add
   )
   (table 10 anyfunc)