librustc: Forbid chained imports and fix the logic for one-level renaming imports
librustc: Remove `fn@`, `fn~`, and `fn&` from librustc. rs=defun
libsyntax: Remove `fn@`, `fn~`, and `fn&` from libsyntax. rs=defun
libstd: Remove `@fn`, `~fn`, and `&fn` from libstd. rs=defun
libcore: Remove `@fn`, `~fn`, and `&fn` from libcore. rs=defun
test: Remove `&fn`, `@fn`, and `~fn` from the test suite. rs=defun
librustdoc: Remove `fn&`, `fn~`, and `fn@` from compiletest, fuzzer, rustdoc, and rt. rs=defun
librustc: Stop parsing `fn&`, `fn~`, and `fn@`