Bug 24289 – Adopt the "rund" tool as a much faster alternative to "rdmd"

Right now the performance of "rdmd" is much worse than the performance of "go run" when invoked to run a freshly edited source file in a script-like fashion. But https://github.com/dragon-lang/rund/ is able to reduce the performance gap and make D language much more attractive and competitive. I propose to add "rund" to https://github.com/dlang/tools/ and distribute it as a standard part of the DMD compiler binary releases. Also update the documentation at https://dlang.org/rdmd.html to recommend "rund" instead of "rdmd". Or alternatively update the "rdmd" tool itself to address its performance problems. Essentially consider bringing back the reverted https://github.com/dlang/tools/pull/271 Extra links: * https://forum.dlang.org/thread/[email protected] * https://forum.dlang.org/thread/[email protected]