libstd: Rename libcore to libstd and libstd to libextra; update makefiles.

This only changes the directory names; it does not change the "real"
metadata names.
librustc: Rename core injection to std injection
libextra: Rename the actual metadata names of libcore to libstd and libstd to libextra
librustc: Change `std` to `extra` throughout libsyntax and librustc
librustc: Make `std_inject` inject the name `std`, not `core`
libfuzzer: Rename `std` to `extra`.
librustdoc: Rename `std` to `extra`.
fuzzer: Change `core` to `std` in the fuzzer
syntax: Change syntax extensions to expand to `std::foo` instead of `core::foo`
test: Update tests to use the new syntax.
test: Update tests and import the prelude in some more places.
libstd: Fix merge fallout.
libsyntax: Fix more merge fallout.
libextra: Allow unnecessary allocations
librustc: Add some missing `use core::prelude::*;` in the test cases
etc: Fix test combiner for core->std/std->extra change. rs=burningtree
libextra: Add missing core::prelude import. rs=burningtree
more testing fallout from core->std/std->extra move
rustc: Check for null before dropping ~objects. #5192
auto merge of #6704 : tedhorst/rust/rename_lib_fixes, r=graydon

This passes make check now.