table construction: internal error, and lack of guard

[shriram]

This program works fine:

include tables

[table-from-rows:
    [raw-row: {"A"; 5}, {"B"; 7}, {"C"; 8}],
    [raw-row: {"A"; 1}, {"B"; 2}, {"C"; 3}]]

but a natural bug is to produce this instead:

[table-from-rows:
  [list:
    [raw-row: {"A"; 5}, {"B"; 7}, {"C"; 8}],
    [raw-row: {"A"; 1}, {"B"; 2}, {"C"; 3}]]]

(e.g., through map).

Not only does this not produce a graceful error, it says to report a bug:

[blerner]

Ugh there are actually missing annotations here:

1. https://github.com/brownplt/pyret-lang/blob/horizon/src/arr/trove/tables.arr#L103 calls https://github.com/brownplt/pyret-lang/blob/horizon/src/arr/trove/tables.arr#L91, which does not annotate its argument as a RawArray.
2. Even if it did, that would be a shallow check, so it would need to be RawArray%(check-that-they-really-are-rows)

I'm not at my coding computer right now; @ds26gte can you take this one? Add a type definition and/or predicate in table.js to check RawArrayOfRows, akin to how the image library does a list of colors (https://github.com/brownplt/code.pyret.org/blob/horizon/src/web/js/trove/internal-image-typed.js#L314), and use it to annotate table-from-raw-array.

[ds26gte]

Please review that PR #1759 does what's needed.