fix #5934 – have tested that Base.rat doesn't hang for any Float32.

This code is now truly independent of word-size, although the size
of integer that it uses is native. However, since it doesn't ever
use integer values larger than 2^24, the computation is the same on
32- or 64-bit systems. Scanning through all Float32 inputs takes
about ten minutes; haven't tested all Float64 values, of course.
Merge branch 'master' of github.com:JuliaLang/julia
slight clarification of quadgk docs, as discussed in JuliaLang/ODE.jl#13
Wrap combinatorics memoization caches in let blocks
Merge pull request #5956 from JuliaLang/kms/comb_minor_cleanup

Wrap combinatorics memoization caches in let blocks
Fix performance regression from c330b1d64efa566911cdd20921ca9419e58bacf9

StoredArray is more specific than AbstractVector{T}, so we were
performing unnecessary copies when calling `int` etc. This caused a
regression on the add1 benchmark, since we call `int` in `getindex`.
Fix #5951