auto merge of #10803 : vmx/rust/integer-decode, r=cmr

The `integer_decode()` function decodes a float (f32/f64)
into integers containing the mantissa, exponent and sign.

It's needed for `rationalize()` implementation of #9838.

The code got ported from ABCL [1].

[1] http://abcl.org/trac/browser/trunk/abcl/src/org/armedbear/lisp/FloatFunctions.java?rev=14465#L94

I got the permission to use this code for Rust from Peter Graves (the ABCL copyright holder) . If there's any further IP clearance needed, let me know.