rewrote nimeval.nim; added tcompilerapi example to show how the compiler can be used as an API

changelog.md

@@ -78,6 +78,11 @@
 - Added the parameter ``val`` for the ``CritBitTree[int].inc`` proc.
 - An exception raised from ``test`` block of ``unittest`` now show its type in
   error message
+- The ``compiler/nimeval`` API was rewritten to simplify the "compiler as an
+  API". Using the Nim compiler and its VM as a scripting engine has never been
+  easier. See ``tests/compilerapi/tcompilerapi.nim`` for an example of how to
+  use the Nim VM in a native Nim application.
+
 
 ### Language additions