Fix Float#abs to return self (#16655)

This fixes a regression from #15814 which unintentionally changed the return type of `Float32#abs` to `Float64`. Previously, it returned `Float32`.

`Math.copysign(self, 1)` falls back to the overload without restrictions which converts both arguments to `Float64` and thus returns `Float64`.

This patch fixes that by explicitly targeting the `Math.copysign(Float32, Float32)` overload for `Float32#abs`.
`Float64#abs` is not broken, but still applies an unnecessary integer-to-float conversion when we should be using a float right away.

I discovered this type mismatch while researching #13389

Author: straight-shoota

src/float.cr

@@ -227,8 +227,8 @@ struct Float32
     (unsafe_as(UInt32) ^ 0x80000000_u32).unsafe_as(Float32)
   end
 
-  def abs
-    Math.copysign(self, 1)
+  def abs : Float32
+    Math.copysign(self, 1.0_f32)
   end
 
   # Returns `-1` if the sign bit of this float is set, `1` otherwise.
@@ -448,8 +448,8 @@ struct Float64
     (unsafe_as(UInt64) ^ 0x80000000_00000000_u64).unsafe_as(Float64)
   end
 
-  def abs
-    Math.copysign(self, 1)
+  def abs : Float64
+    Math.copysign(self, 1.0)
   end
 
   # Returns `-1` if the sign bit of this float is set, `1` otherwise.