AK: Remove String-from-StringView optimization

We had an unusual optimization in AK::StringView where constructing
a StringView from a String would cause it to remember the internal
StringImpl pointer of the String.

This was used to make constructing a String from a StringView fast
and copy-free.

I tried removing this optimization and indeed we started seeing a
ton of allocation traffic. However, all of it was due to a silly
pattern where functions would take a StringView and then go on
to create a String from it.

I've gone through most of the code and updated those functions to
simply take a String directly instead, which now makes this
optimization unnecessary, and indeed a source of bloat instead.

So, let's get rid of it and make StringView a little smaller. :^)

AK/String.cpp

@@ -38,10 +38,7 @@ namespace AK {
 
 String::String(const StringView& view)
 {
-    if (view.m_impl)
-        m_impl = *view.m_impl;
-    else
-        m_impl = StringImpl::create(view.characters_without_null_termination(), view.length());
+    m_impl = StringImpl::create(view.characters_without_null_termination(), view.length());
 }
 
 bool String::operator==(const FlyString& fly_string) const