emitbc: Correct buffer sizes for varlen int encoding.

Assuming we have truncating (floor) division, way to do ceiling division
by N is to use formula (x + (N-1)) / N. Specifically, 63 bits, if stored
7 bits per byte, require exactly 9 bytes. 64 bits overflow that and require
10 bytes.
__import__: Catch relative import attempts and throw NotImplementedError.