Rhai Bevy API

[makspll]

Now that the foundational work is done the Rhai API can be written as well, the work needed is as follows:

• Rhai implementor in bevy_mod_scripting_derive which generates Rhai<Type> Wrappers similarly to the Lua implementor
• Traits akin to LuaProxyable ToLuaProxy and FromLuaProxy
• std implementations of primitives + Option and Vec for the traits at least

[makspll]

Closed in #40