Eliminate runtime bash dependency

[ii8]

This removes mltons runtime bash dependency and uses only portable shell script instead. Bash is still needed for building.

Benefits are:

• Packagers of mlton may not notice the bash dependency because they happen to have bash installed anyway, so their mlton package runs fine for them, but will break for somebody else who does not have bash installed.
• Simplifies installation of binary distribution, for example in minimal docker containers where bash is not preinstalled.
• Smaller downloads and disk space requirement for users who do not already use bash.
• More easily portable to new platforms where bash may not be available at all which is very desirable for a compiler.

[MatthewFluet]

I understand the motivation and it seems simple enough. Though, I wouldn't advocate for aggressively converting all of the scripts.

[pclayton]

The ShellCheck utility is useful for identifying a wide range of issues that can affect portability/robustness.