Minor refactoring of TSymbolTable.

Used pointers for precision-stack entries as well to avoid unnecessarily re-allocating PrecisionStackLevel whenever the vector needs to resize.

Added a scoped class to properly restore symbol-table level after each compile.

R=kbr@chromium.org

Review URL: https://codereview.appspot.com/12583047
diff --git a/src/compiler/SymbolTable.cpp b/src/compiler/SymbolTable.cpp
index f9daad4..1132386 100644
--- a/src/compiler/SymbolTable.cpp
+++ b/src/compiler/SymbolTable.cpp
@@ -246,3 +246,9 @@
 
     return 0;
 }
+
+TSymbolTable::~TSymbolTable()
+{
+    while (table.size() > 0)
+        pop();
+}