make auto allocators optional

[electronstudio]

make sure advanced users have option to specify their own allocators to improve performance - the auto ones should not be mandatory anywhere

[electronstudio]

for constructors and raylib functions that allocate you can now specify your own arena allocator. the only auto arena is when converting string arguments to function calls. i guess this should be replaced by:

if function call is one that has string arguments:

• if no arena supplied, then create a local arena, do function call, then destroy it (i.e. try with resources) (but is this any quicker than just using an auto arena that goes out of scope? i doubt it)
• elif arena supplied, use it. (but in most cases this would not be what the user should do because memory isnt freed, unless performance of creating arena is problem.)

performance impact of all this is probably so tiny it's not a priority

[electronstudio]

The auto arena for string arguments does actually reduce bunnymark by 4000 when using DrawText()!

Another alternative: allocate temp memory once on startup and then re-use it. not threadsafe. might need custom arena/allocator or something.

[electronstudio]

local arena for string arguments done