Implement idiomatic Dart representations of some types

[filleduchaos]

Nobody actually wants to work with Pointer<Utf8>s.

There needs to be the option to use those if you want though.

[filleduchaos]

Changing the scope of this to handle types that have a(n easy) built-in Dart analogue but aren't (yet) automatically wrapped/unwrapped by dart:ffi.

Targeted types:

• Booleans
• Strings
• Enums