Native Memory Helpers, take 2

[craiglabenz]

Adds helpers for converting between Dart and native memory for the following scenarios:

• Float32List <-> Pointer<Float>
• Uint8List <-> Pointer<Char>
• String <-> Pointer<Char>
• List<String> <-> Pointer<Pointer<Char>>

Some of this functionality already existed, and some of it is new to this PR. Where the functionality already existed, the implementations were consolidated on extension methods instead of free-floating functions. Also, methods to convert from native memory to Dart types now return nullable types instead of throwing exceptions on null pointers.

Replaces #18

cc @Piinks

[craiglabenz]

CI is failing because this refactors code that was in use elsewhere, but I want to make sure the utilities are correct before propagating their changes everywhere. However, those updates are quite trivial.

[craiglabenz]

Obsolete.