[feature request] Strict mode

[johnkerl]

Miller is designed around heterogeneous data -- $z = $x + $y should not produce an error if a particular record lacks fields $x, $y, or both. See also https://miller.readthedocs.io/en/latest/reference.html#reference-null-data

However, this very flexibility can be a hindrance -- it can mask typos in confusing ways.

One option is a strict mode which would implicit wrap variable/record-field reads in asserting_present functions.

Great discussion with @torbiak is on #431.

Note for scheduling: I've no plans to work this into the C implementation (Miller 5) -- this will fit nicely into the Go port (Miller 6).