Added 'Toy_String' to 'Toy_Value' structure, read more

To help with storing strings within tables, I've replaced the unused
'_padding' member of 'Toy_String' with 'cachedHash', which is set to
zero on string allocation.

The hash of a string isn't generated and stored until it's actually
needed, as the rope pattern means not every string needs a hash -
hopefully this will save unnecessarily wasted time.

When a hash of a string is needed, the hashing function first checks to
see if that string already has one, and if so, returns it. Again, less
time wasted.

When generating a new string hash, the hashing function takes the
string's type into account, as node-based strings first need their
contents assembled into a simple char buffer.

Other changes include:

* Changed 'TOY_VALUE_TO_*' to 'TOY_VALUE_FROM_*'
* Changed 'TOY_VALUE_IS_EQUAL' to 'TOY_VALUES_ARE_EQUAL'
* Added a missing '#pragma once' to 'toy_print.h'

source/toy_string.h

@@ -1,3 +1,5 @@
+#pragma once
+
 #include "toy_common.h"
 
 #include "toy_bucket.h"
@@ -13,8 +15,7 @@ typedef struct Toy_String {             //32 | 64 BITNESS
 
 	unsigned int length;                //4  | 4
 	unsigned int refCount;              //4  | 4
-
-	int _padding;                       //4  | 4
+	unsigned int cachedHash;            //4  | 4
 
 	union {
 		struct {
@@ -52,3 +53,4 @@ TOY_API int Toy_getStringRefCount(Toy_String* str);
 TOY_API char* Toy_getStringRawBuffer(Toy_String* str); //allocates the buffer on the heap, needs to be freed
 
 TOY_API int Toy_compareStrings(Toy_String* left, Toy_String* right); //return value mimics strcmp()
+