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

tests/compilerapi/exposed.nim

@@ -0,0 +1,3 @@
+
+proc addFloats*(x, y, z: float): float =
+  discard "implementation overriden by tcompilerapi.nim"

tests/compilerapi/myscript.nim

@@ -0,0 +1,7 @@
+
+import exposed
+
+echo "top level statements are executed!"
+
+proc hostProgramRunsThis*(a, b: float): float =
+  result = addFloats(a, b, 1.0)

tests/compilerapi/tcompilerapi.nim

@@ -0,0 +1,36 @@
+discard """
+  output: '''top level statements are executed!
+2.0
+'''
+"""
+
+## Example program that demonstrates how to use the
+## compiler as an API to embed into your own projects.
+
+import "../../compiler" / [ast, vmdef, vm, nimeval]
+import std / [os]
+
+proc main() =
+  let std = findNimStdLib()
+  if std.len == 0:
+    quit "cannot find Nim's standard library"
+
+  var intr = createInterpreter("myscript.nim", [std, getAppDir()])
+  intr.declareRoutine("*", "exposed", "addFloats", proc (a: VmArgs) =
+    setResult(a, getFloat(a, 0) + getFloat(a, 1) + getFloat(a, 2))
+  )
+
+  intr.evalScript()
+
+  let foreignProc = selectRoutine(intr, "hostProgramRunsThis")
+  if foreignProc == nil:
+    quit "script does not export a proc of the name: 'hostProgramRunsThis'"
+  let res = intr.callRoutine(foreignProc, [newFloatNode(nkFloatLit, 0.9),
+                                           newFloatNode(nkFloatLit, 0.1)])
+  if res.kind == nkFloatLit:
+    echo res.floatVal
+  else:
+    echo "bug!"
+  destroyInterpreter(intr)
+
+main()