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.h

@@ -3,6 +3,7 @@
 //general utilities
 #include "toy_common.h"
 #include "toy_console_colors.h"
+#include "toy_print.h"
 
 //basic structures
 #include "toy_value.h"
@@ -21,3 +22,4 @@
 #include "toy_parser.h"
 #include "toy_bytecode.h"
 #include "toy_vm.h"
+