Fix issue with unsigned integers

Fixes https://github.com/dlozeve/bqn-npy/issues/1.

The spec says that •bit._cast should support 32-bit uints [1], but
CBQN does not [2].

To get around this, we just use signed integers, which works for
numbers smaller than 2^31. Above that, it will wrap around
and (probably) fail, but CBQN does not support them anyway (except by
converting them to doubles).

From Marshall:

> All of the provided functions at this time are identical on i32 and
> u32, because they work (mod 2^32) and the only difference between
> those formats is to add or subtract 2^32 from numbers outside the
> range [0,2^31).
> If you want to run on u32s stored directly as numbers in BQN (they
> won't fit in i32 and will be stored as double!) you can •bit._cast
> those into a signed format at the beginning and then back out at the
> end.

[1] https://mlochbaum.github.io/BQN/spec/system.html#bitwise-operations
[2] https://matrix.to/#/!EjsgbQQNuTfHXQoiax:matrix.org/$PL2AYNlLQuYhPftdt7yw7rW9heiVEEeATte2zOt2BYk

tests.bqn

@@ -1,8 +1,8 @@
 ⟨SaveNpy,LoadNpy⟩←•Import"npy.bqn"
 
 floatArr←2‿3•rand.Range 0
-intArr←50-˜2‿3•rand.Range 100
-uintArr←2‿3•rand.Range 100
+intArr←(2⋆31)-˜2‿3•rand.Range 2⋆32
+uintArr←2‿3•rand.Range 2⋆31
 
 "test_float.npy"SaveNpy floatArr
 "test_int.npy"SaveNpy intArr