[GSoC] Initial numbers on xlsx load time

Hi there,

I just did a quick comparison of the load time of one large-ish xlsx
document that contains a bunch of number and string cells just to see
how much difference Daniel's initial changes have made.  Here is the
result: on the latest master branch, the file loaded in 52 seconds,
whereas the build from the feature/gsoc-calc-perf branch the number
was down to 32 seconds.  The file only contains one sheet, with 8
column by 300000 row cells.

I think it's safe to say that we are making progress. ;-)

Kohei