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_bytecode.h

@@ -11,3 +11,4 @@ typedef struct Toy_Bytecode {
 
 TOY_API Toy_Bytecode Toy_compileBytecode(Toy_Ast* ast);
 TOY_API void Toy_freeBytecode(Toy_Bytecode bc);
+