Fix panic on duplicated top-level idents in record destructuring

[matthew-healy]

Previously Nickel would panic when encountering code like this:

let f = fun { x, x } = x in f { x = 1 }

This commit fixes that by checking each destructured record pattern for duplciated identifiers at parsing time, and returning an error if any are encountered.

Note, however, that in order to preserve backwards compatibility with Nickel 1.0, the following code is still valid (and returns 1):

let f = fun { x = { y }, z = { y } } => y
in f { x = { y = 1 }, z = {  y = 2 } }

Fixes #1318.

[vkleen]

The duplicate identifiers don't even need to be nested for the previously working example:

fun { foo = x, bar = x } => x

picks out the value of foo.

[matthew-healy]

@vkleen Ah, good point! That will also still work fine after this change, though it'd be great to fix it when we next allow breaking changes.