Unicode text manipulation

[fmease]

Provide types for UTF-8 encoded strings (currently named Text), strings in the ASCII range (Byte-String maybe) and several “character”-like entities:

• ASCII character / UTF-8 Code Unit: Naming unclear expect that we should stay away from Char! (Rust's overloaded u8), maybe Byte-Character
• Unicode Scalar (Rust's char, Swift's Unicode.Scalar): Naming unclear
• Extended Grapheme Cluster (Swift's Character): Naming unclear, maybe Char or Character

All of those types should allow construction via "text" literals (see #126). E.g. Text."test", Char."ä", Scalar."ä". What about normalization? Swift automatically normalizes (literals, any string, …?).