MINOR: tools: add a function varint_bytes() to report the size of a varint

MINOR: tools: add a function varint_bytes() to report the size of a varint

It will sometimes be useful to encode varints to know the output size in
advance. Two versions are provided, one inline using a switch/case construct
which will be trivial for use with constants (and will be very fast albeit
huge) and one function iterating on the number which is 5 times smaller,
for use with variables.