LibIPC: Add support for transferring doubles over IPC messages

I'm still wondering why nobody did this yet :^) Also changes the use of
unions for the more cleaner / less undefined AK::bit_cast.

Userland/Libraries/LibIPC/Decoder.cpp

@@ -78,6 +78,12 @@ bool Decoder::decode(float& value)
     return !m_stream.handle_any_error();
 }
 
+bool Decoder::decode(double& value)
+{
+    m_stream >> value;
+    return !m_stream.handle_any_error();
+}
+
 bool Decoder::decode(String& value)
 {
     i32 length = 0;

Userland/Libraries/LibIPC/Decoder.h

@@ -41,6 +41,7 @@ public:
     bool decode(i32&);
     bool decode(i64&);
     bool decode(float&);
+    bool decode(double&);
     bool decode(String&);
     bool decode(ByteBuffer&);
     bool decode(URL&);

Userland/Libraries/LibIPC/Encoder.cpp

@@ -1,9 +1,11 @@
 /*
  * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, kleines Filmröllchen <malu.bertsch@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/BitCast.h>
 #include <AK/ByteBuffer.h>
 #include <AK/String.h>
 #include <AK/URL.h>
@@ -98,12 +100,14 @@ Encoder& Encoder::operator<<(i64 value)
 
 Encoder& Encoder::operator<<(float value)
 {
-    union bits {
-        float as_float;
-        u32 as_u32;
-    } u;
-    u.as_float = value;
-    return *this << u.as_u32;
+    u32 as_u32 = bit_cast<u32>(value);
+    return *this << as_u32;
+}
+
+Encoder& Encoder::operator<<(double value)
+{
+    u64 as_u64 = bit_cast<u64>(value);
+    return *this << as_u64;
 }
 
 Encoder& Encoder::operator<<(char const* value)

Userland/Libraries/LibIPC/Encoder.h

@@ -37,6 +37,7 @@ public:
     Encoder& operator<<(i32);
     Encoder& operator<<(i64);
     Encoder& operator<<(float);
+    Encoder& operator<<(double);
     Encoder& operator<<(char const*);
     Encoder& operator<<(StringView const&);
     Encoder& operator<<(String const&);