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

npy.bqn

@@ -4,7 +4,11 @@ lf←@+10
 magicString←(@+147)∾"NUMPY"
 
 # Type conversions
-dtypes‿formats←⟨"<f8","<i4","<u4"⟩‿⟨64‿'f',32‿'i',32‿'u'⟩
+
+# CBQN does not support other widths than 1 for 'u'. However, signed
+# and unsigned integers seem to have the same behavior for numbers
+# below 2⋆31.
+dtypes‿formats←⟨"<f8","<i4","<u4"⟩‿⟨64‿'f',32‿'i',1‿'u'⟩
 DtypeToFormat←{(⊑dtypes⊐<𝕩)⊑formats}
 ArrayToBytes←{⟨DtypeToFormat 𝕨,8‿'c'⟩•bit._cast 𝕩}
 ArrayFromBytes←{⟨8‿'c',DtypeToFormat 𝕨⟩•bit._cast 𝕩}