r? @brson 

Rewrote the last pull request (#4859) to not require on llvm for core.

Also fixes #2269.

and then adjust code to match. rs=unsound (will review post-landing)

rs=unsound

and then adjust code to match. rs=unsound (will review post-landing)

r? @nikomatsakis 

When you made the change to fix kinds in recursion, you stopped looking in the master cache. This patch fixes it.

Update of #4862

This reverts commit a8f039a0.

core: Rewrite last_os_error in Rust for unix and provide access to errno (unix) and GetLastError (windows).

rust-mode.el uses the 'cl macros, so it should actually require them

Performance before:

    std::treemap::TreeMap
                   sequential_ints 0.083971 s
                       random_ints 0.095861 s
                       delete_ints 0.083931 s
                sequential_strings 0.278272 s
                    random_strings 0.240286 s
                    delete_strings 0.173581 s

Performance after:

    std::treemap::TreeMap
                   sequential_ints 0.083297 s
                       random_ints 0.097644 s
                       delete_ints 0.052602 s
                sequential_strings 0.287326 s
                    random_strings 0.242372 s
                    delete_strings 0.142269 s

Performance before:

    std::treemap::TreeMap
                   sequential_ints 0.151877 s
                       random_ints 0.160926 s
                       delete_ints 0.08694 s
                sequential_strings 0.316458 s
                    random_strings 0.290778 s
                    delete_strings 0.169892 s

Performance after:

    std::treemap::TreeMap
                   sequential_ints 0.083971 s
                       random_ints 0.095861 s
                       delete_ints 0.083931 s
                sequential_strings 0.278272 s
                    random_strings 0.240286 s
                    delete_strings 0.173581 s