CLI bundle command: add debug_mode and kernel flags

[plafer]

Context: #1445 (comment) and #1445 (comment)

Currently, the bundle CLI command

1. creates a Library from a directory, and
2. automatically includes debug symbols.

It would be great if both were configurable: a --kernel flag would create a KernelLibrary instead of a Library, and a --debug flag would specify to include debug symbols (otherwise they wouldn't be).

[bitwalker]

I'd suggest making --debug the default, and require --debug=false or --no-debug (or something to that effect) to disable that default. Just like cargo/rustc do debug builds by default, and require specifically requesting release builds. We should probably encourage building with debug information, because without it, programs are exceedingly difficult to troubleshoot when something goes wrong in a procedure without that information.