feat: Simplify pipe / eliminate $input

[bjchambers]

Summary
Currently, <a> | <b> is implemented as let $input = <a> in <b>, with some special rules for inferring $input for ommitted arguments. This allows for interesting uses -- such as referencing the left hand side in a deeply nested location on the right hand side.

A simpler approach inspired by PRQL would be to have a simple rule like <a> | f(<b...>) always becomes f(<b...>, <a>) (the left hand side becomes the last argument to the function on the right.

This would allow us to:

1. Eliminate the rules for inferring $input as an argument. In turn, this would simplify error messages when an argument is missing (instead of inferring $input and then reporting it isn't used).
2. Simplify the rules for implementing pipe.
3. Eliminate the concept of $input from what we need to document (in favor of the simpler explanation of pipe). Any examples that need it can be re-written (and likely clarified) using an explicit let binding and name.
4. Eliminate the case of the left hand side of a pipe not being used.