'compile' option to compile in local context

[s-hadinger]

During compilation to .bec files, now the compilation is done in local context rather than global context.

Example, when solidifying:

class AClass
end

return AClass

Before, the .bec file would always create AClass global object; now it returns the AClass object but does not create an entry in globals anymore. This is useful to have the ability to unload in the future.

Also add an option to Berry native compile(code_or_filename:string [, mode:string, islocal:bool]) -> function. Now if islocal is true, the compilation is done in local context, or by default in global context.