work around MinGW linker bug for ASLR on Windows

Closes #16514
Explain modulo in the guide.

Fixes #15954
Make comment about small bias in %.

Fixes #16354.
Fix underflow bug in core::str::Searcher::new for haystacks of length < 20
Add a test for the fix of issue 16589
auto merge of #16585 : steveklabnik/rust/random_remarks, r=pcwalton

Fixes #15954 and #16354.
auto merge of #16588 : thestinger/rust/aslr, r=pcwalton

Closes #16514
auto merge of #16590 : nham/rust/str_searcher_underflow, r=alexcrichton

This incidentally fixes #16589, because it will cause `MatchIndices` to use `NaiveSearcher` instead of `TwoWaySearcher`, but I'm not sure #16589 should be closed until the underlying problem in `TwoWaySearcher` is found.