Remove StrBuf

[bugadani]

StrBuf has two responsibilities:

• StrBuf is mostly used to turn a *const u8 into a &str. While this is completely unsafe to do, using the same assumptions it's much more efficient to turn the pointer into a slice instead of copying the string.
• StrBuf is used as a string builder. The string is assembled in a local buffer, then copied to the destination pointer unconditionally. I've replaced this with direct writing into the destination pointer.

[bjoernQ]

LGTM