coerce key in map lookup

[mccanne]

Right now, map lookups return missing if the lookup value does not exactly match the type of the map key. This should be more robust with coercion etc.

[philrz]

Here's my attempt at a repro with Zed commit 7396ea9.

$ cat repro.zson 
{
    m: |{
        1: "a",
        2: "b",
        3: "c" 
    }|
}

$ zq -version
Version: v1.2.0-97-g7396ea9c

$ zq -Z 'yield m[2]' repro.zson
"b"

$ zq -Z 'yield m[2.0]' repro.zson
error("missing")

$ zq -Z 'yield m[uint64(2)]' repro.zson
error("missing")

I found what looks like a separate map bug while reproducing this one, so I've opened #4167 to track it.