Fix Int/BigInt pivot

We've been wrongly representing large ints as BigInt, causing them to
  behave differently in the VM through builtins like 'serialise_data'.

  Indeed, we expect anything that fits in 8 bytes to be encoded as Major
  Type 0 or 1. But we were switching to encoding as Major type 6
  (tagged, PosBigInt, NegBigInt) for much smaller values! Anything
  outside of the range [-2^32, 2^32-1] would be treated as big int
  (positive or negative).

  Why? Because we checked whether a value i would fit in an i64, and if
  it didn't we treated it as big int. But the reality is more subtle...
  Fortunately, Rust has i128 and the minicbor library implements TryFrom
  which enforces that the value fits in a range of [-2^64, 2^64 - 1], so
  we're back on track easily.

examples/acceptance_tests/094/lib/foo.ak

@@ -0,0 +1,9 @@
+use aiken/builtin
+
+test u32_boundary_down() {
+  builtin.serialise_data(0xdeadbeefdeadbeef) == #"1bdeadbeefdeadbeef"
+}
+
+test u32_boundary_up() {
+  builtin.serialise_data(-0xdeadbeefdeadbeef) == #"3bdeadbeefdeadbeee"
+}