Fix UTF8 strings inside structs

[rlabrecque]

When marshaling structs containing UTF8 strings we need to explicitly marshal them ourselves.

Idea taken from:
@Akarinnnnn
#339

[rlabrecque]

Feedback appreciated!

We can't use Span unfortunately because Unity doesn't support it yet.

We can't use an ICustomMarshaler because Unity has had pretty bad issues with them in the past (crashing when a string is between 96 and 127 characters long for example.)

[Akarinnnnn]

Seems good to merge. Consider to merge?

[rlabrecque]

It's in! I regenerated against 1.49 and swapped to get {} set {}, instead of get => set => for compatibility with Unity 4.7 (hopefully no one's using that still?)