Replace calls to StaticArray(UInt8, 1)#unsafe_as(UInt8) (#14685)

It's unnecessary to use `unsafe_as` to retrieve the value of a single-item `StaticArray`.
However, the entire `StaticArray` is unnecessary because we just need a slice pointing to the location of the value.
The resulting machinecode should be identical in all three variants.

src/crystal/system/unix/getrandom.cr

@@ -62,9 +62,9 @@ module Crystal::System::Random
     init unless @@initialized
 
     if @@getrandom_available
-      buf = uninitialized UInt8[1]
-      getrandom(buf.to_slice)
-      buf.unsafe_as(UInt8)
+      buf = uninitialized UInt8
+      getrandom(pointerof(buf).to_slice(1))
+      buf
     elsif urandom = @@urandom
       urandom.read_byte.not_nil!
     else

src/crystal/system/wasi/random.cr

@@ -7,8 +7,8 @@ module Crystal::System::Random
   end
 
   def self.next_u : UInt8
-    buf = uninitialized UInt8[1]
-    random_bytes(buf.to_slice)
-    buf.unsafe_as(UInt8)
+    buf = uninitialized UInt8
+    random_bytes(pointerof(buf).to_slice(1))
+    buf
   end
 end

src/crystal/system/win32/random.cr

@@ -8,8 +8,8 @@ module Crystal::System::Random
   end
 
   def self.next_u : UInt8
-    buf = uninitialized UInt8[1]
-    random_bytes(buf.to_slice)
-    buf.unsafe_as(UInt8)
+    buf = uninitialized UInt8
+    random_bytes(pointerof(buf).to_slice(1))
+    buf
   end
 end