refs #68; turn some macros to CT procs; fix #112 (doc fixes) (#102)

* refs #68 macro=>proc cSkipSymbol
* refs #68 macro=>proc cDebug
* refs #68 macro=>proc cDisableCaching
* refs #68 notes for cDefine
* refs #68 macro=>proc cAddSearchDir ; improve some runnableExamples
* refs #68 macro=>proc cAddStdDir
* $projpath/include => testsIncludeDir() everywhere
* disable $projpath interpolation (error prone)
* fix tests
* make nim doc part of test suite; fix for 0.19.2

tests/tnimterop_c.nim

@@ -1,12 +1,14 @@
 import std/unittest
 import nimterop/cimport
+import nimterop/paths
 
-cDebug()
-cDisableCaching()
+static:
+  cDebug()
+  cDisableCaching()
+  cAddSearchDir testsIncludeDir()
 
 cDefine("FORCE")
-cIncludeDir "$projpath/include"
-cAddSearchDir "$projpath/include"
+cIncludeDir testsIncludeDir()
 cCompile cSearchPath("test.c")
 
 cPlugin:
@@ -137,8 +139,6 @@ var
   k: uKernel
   kp: Kernel
 
-cAddStdDir()
-
 ## failing tests
 when false:
   static: # Error: undeclared identifier: 'foobar1'