Add concept of String in SwIPC

[roblabla]

SwIPC has no concept of Strings, and simply uses bytes or array or buffer whenever it needs one. This is extremely annoying, because rust separates those two concepts at the type level.

My first naive attempt at adding string support was quickly met with many difficulties, most notably that array/buffer expect sized raw data.

Upstream tracking issue: reswitched/SwIPC#28

[roblabla]

Something I found out recently: bitmask 0x10 in the IPC "type" means "Null-terminated string"