See my other comments on this thread for a simple implementation of a bijective big-endian prefix varint encoder. You may or may not want a bijective encoding, and probably want little-endian. I'm just used to writing big-endian encoders (for lexographical sorting reasons), so that was faster for me to whip up a demonstration of a bijective encoder.

A real implementation would use a switch statement instead of a loop. One might use a lookup table or a few instructions of inline assembly to calculate the number of leading ones in the first byte, and switch on that.