change l -> ℓ for integer local variables, for legibility

As @StefanKarpinski remarked in commit d15ce97f9fa026ca1c97299dce829d48869c8838, the use of `l` as a variable is often a bit unfortunate from a legibility standpoint, since it is easily confused with `1`.  This patch changes most such usages in the Julia source code for integer local variables to `ℓ` instead (LaTeX [\ell](http://www.fileformat.info/info/unicode/char/2113/index.htm)).  cc: @jiahao