Incremental compilation (IC): Improvements (#11881)

* IC: C codegen is aware of IC
* manual: minor change to make VSCode's RST plugin render it properly
* IC: minor refactoring
* testament: code refactorings
* rodutils: removed dead code
* IC: always build the compiler with the IC feature
* IC: C codegen improvements
* IC: implement the undocumented -d:nimMustCache option for testing purposes
* IC: added first basic tests
* IC: extensive testing of the deserialization feature
* testament: refactoring; better IC tests
* IC: removes 'nimMustCache' flag; readonly does the same
* testament: minor refactoring
* update Nimble version
* testament: removed dead code and imports; IC: added simple test
* IC: progress

tests/ic/tgenerics.nim

@@ -0,0 +1,39 @@
+discard """
+  output: "bar"
+  disabled: "true"
+"""
+
+import tables
+
+var tab: Table[string, string]
+
+tab["foo"] = "bar"
+echo tab["foo"]
+
+#!EDIT!#
+
+discard """
+  output: "bar 3"
+"""
+
+import tables
+
+var tab: Table[string, string]
+var tab2: Table[string, int]
+
+tab["foo"] = "bar"
+tab2["meh"] = 3
+echo tab["foo"], " ", tab2["meh"]
+
+#!EDIT!#
+
+discard """
+  output: "3"
+"""
+
+import tables
+
+var tab2: Table[string, int]
+
+tab2["meh"] = 3
+echo tab2["meh"]

tests/ic/thallo.nim

@@ -0,0 +1,28 @@
+discard """
+  output: "Hello World"
+"""
+
+const str = "Hello World"
+echo str
+
+# Splitters are done with this special comment:
+
+#!EDIT!#
+
+discard """
+  output: "Hello World B"
+"""
+
+const str = "Hello World"
+echo str, " B"
+
+#!EDIT!#
+
+discard """
+  output: "Hello World C"
+"""
+
+const str = "Hello World"
+var x = 7
+if 3+4 == x:
+  echo str, " C"