Document identifier index limit

[travisbrown]

The Haskell implementation will happily parse an arbitrarily large index on an identifier (although it looks like something's overflowing either in the representation or the encoding):

$ dhall encode --json <<< "x @ 9223372036854775808"
[
    "x",
    -9223372036854775808
]

We stop at Long.MaxValue:

scala> import org.dhallj.parser.DhallParser
import org.dhallj.parser.DhallParser

scala> DhallParser.parse("x @ 9223372036854775807")
res0: org.dhallj.core.Expr.Parsed = x@9223372036854775807

scala> DhallParser.parse("x @ 9223372036854775808")
java.lang.NumberFormatException: For input string: "9223372036854775808"
  at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:68)
  at java.base/java.lang.Long.parseLong(Long.java:703)
  ...

I think this is fine, since nobody's ever going to bind 9,223,372,036,854,775,808 xs, so this will never type-check, anyway, but we should document the difference and maybe wrap the NumberFormatException in an ParseException.