Don't enable the serde/alloc feature when alloc is enabled.

It looks like serde still pulls std when only `alloc` is selected.

Cargo.toml

@@ -26,7 +26,7 @@ default         = [ "std" ]
 std             = [ "matrixmultiply", "simba/std", "serde/std" ]
 sparse = [ ]
 debug = [ "approx/num-complex", "rand" ]
-alloc = [ "serde/alloc" ]
+alloc = [ ]
 io = [ "pest", "pest_derive" ]
 compare = [ "matrixcompare-core" ]
 libm = [ "simba/libm" ]