Round-trip failure with certain unsigned int64 values

[beojan]

Jsonnet produces surprising output for certain unsigned int64 values. For example, with this test.jsonnet:

{
    a_uint: 18446744073709551615,
    b_uint: 9223372036854775807
}

jsonnet test.jsonnet produces this output:

{
   "a_uint": 18446744073709551616,
   "b_uint": 9223372036854775808
}

Tagging Framework-R-D/phlex#243.

EDIT: Evidently this is because all numbers are 64-bit floats in Jsonnet.