Fix compile error in prelude/input + rename it

[DestyNova]

I ran into errors like this when trying to call input in the Prelude:

error: No impl found for Extract (Ptr char) usz (ref char)
    buffer#len := '\0'

This PR does the following:

• fixes the compile error by replacing buffer#len assignment in prelude/input with call to array_insert (with help from Ante's Discord chat -- thanks!)
• splits input into two functions: input and read_line, since you'll often want to read a line from stdin without printing a message every time
• adds a check for EOF so programs don't hang forever if empty streams are piped in

[DestyNova]

I see some test failures in CI but not sure how that could happen... will take a look :thinking:

[jfecher]

Looks like the original error from not using array_insert has been fixed. I've also split input into input and read_line as well. I'm closing this since the original error is gone now, but will open a new issue for the bitcast panic if I can reproduce it again.