Adds reusable `for_each_input` method

[zslayton]

Experimenting with ways to cut down the boilerplate needed to set up IO in each command. While different commands take different approaches to handling input (some mmap, some make a Reader, some make a SystemReader, etc), the handling for output it pretty universal.

This patch shows what the change would look like for the symtab filter and to json commands.

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