Fix std.filter and std.filterMap to reject non-boolean predicate results instead of treating them as "false"

[JoshRosen]

This PR fixes a bug where filter() and filterMap() mishandled filter predicate results that weren't booleans: instead of failing with an error, jsonnet treated any non-boolean value as though it was false. For example:

std.filter(function(x) "not a boolean", [1, 2, 3])
// sjsonnet (before): [] (silently treats non-boolean as false)
// jsonnet: RUNTIME ERROR: filter function must return boolean, got: string
// go-jsonnet: RUNTIME ERROR: Unexpected type string, expected boolean

The problem was that evaluator code was checking .isInstanceOf[Val.True] rather than explicitly checking for booleans (very similar to the past issue fixed in #44).

This PR's fix is to update this logic to use .asBoolean , which throws an explicit type error for non-booleans:

sjsonnet.Error: Wrong parameter type: expected Boolean, got string