Add basic Credentials wrapper + ManagedNative mixin

[alextwoods]

Description of changes:

• Add a basic Credentials object that wraps CRT Credentials and conforms to the Aws::Credentials interface.
• Add ManagedNative mixin for classes that manage/wrap a native object.
• Add ByteCursor struct for outputs instead of :string.
• Extend FFI attach_function in our module to provide: 1) error wrapping (with an "unsafe" ! version of the method that does not do error checking) and 2) handle renaming methods in ruby to remove the aws_crt_ prefix. This replaces the Aws::Crt::call method completely!

Note: TruffleRuby currently has an FFI bug - they do not support Struct.by_value. I've +1 and commented on their issue: oracle/truffleruby#2069 but for now am dropping truffleruby from the CI :-(

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[graebm]

reviewed Credentials