Greatly improve /proc/PID/stack by tracing the ebp frame chain.

I also added a generator cache to FileHandle. This way, multiple
reads to a generated file (i.e in a synthfs) can transparently
handle multiple calls to read() without the contents changing
between calls.

The cache is discarded at EOF (or when the FileHandle is destroyed.)

VirtualFileSystem/FileHandle.h

@@ -32,6 +32,8 @@ public:
     void setBlocking(bool b) { m_isBlocking = b; }
 #endif
 
+    ByteBuffer& generatorCache() { return m_generatorCache; }
+
 private:
     friend class VirtualFileSystem;
 
@@ -39,6 +41,8 @@ private:
 
     Unix::off_t m_currentOffset { 0 };
 
+    ByteBuffer m_generatorCache;
+
 #ifdef SERENITY
     int m_fd { -1 };
     bool m_isBlocking { true };