internal/core/adt: add isconcrete builtin func

[bozaro]

This PR implements isconcrete buildin func from #943 proposal.

isconcrete(expr) :: bool

isconcrete reports whether expr resolves to a concrete value, returning true if it does and false otherwise. It is a fatal error if an expression can never evaluate to true.

Motivation: replaces if a.foo != _|_ { field value check without hiding the underlying error.

[fab29p]

With the following file, there is a difference in the behavior of isconcrete and the behavior of != _|_ when doing a cue export. Try by commenting the line with a_1 then the line with a_2. The test on a_1 raises an error.

a: "z"
x: {
    y: 1
    
}

let u = {
    for k, v in x {
        (k): v
    }
}

a_1: isconcrete(u[a])
a_2: u[a] != _|_

[fab29p]

Notice that

• the proposal for exists exhibit the same behavior
• if let u= is replaced by u: then the problem disappear