Completely rewritten keyword completion implementation using another approach

#KT-2816 Fixed

idea/testData/completion/keywords/InCodeBlock.kt

@@ -0,0 +1,27 @@
+fun foo() {
+    <caret>
+}
+
+// EXIST: break
+// EXIST: class
+// EXIST: continue
+// EXIST: do
+// EXIST: false
+// EXIST: for
+// EXIST: fun
+// EXIST: if
+// EXIST: null
+// EXIST: object
+// EXIST: package
+// EXIST: return
+// EXIST: super
+// EXIST: this
+// EXIST: throw
+// EXIST: trait
+// EXIST: true
+// EXIST: try
+// EXIST: val
+// EXIST: var
+// EXIST: when
+// EXIST: while
+// NUMBER: 22