compiler: make compiler an ordinary vlib/compiler module

* Move compiler/ under vlib/compiler/ .

* Add a minimal compiler/main.v driver program.

* Cleanup compiler/main.v .

* Make most compiler tests pass again.

* Apply the fix by @joe-conigliaro , so that the rest of the compiler tests are fixed too.

* Thanks to @avitkauskas, now the vlib/vcompiler/tests/str_gen_test.v test does not need to be special cased anymore.

* Reapply @joe-conigliaro fix for vgen.

vlib/compiler/tests/repl/README.md

@@ -0,0 +1,25 @@
+# V REPL Tests Script
+
+### How to write a new test
+  - Create a new file named `*.repl`
+  - Write the input to be given to REPL
+  - Add `===output===`
+  - Write the output expected
+  
+### Notes
+Keep in mind, that the way V repl works for now, every non empty line
+would cause a new recompilation of the entire repl content that was
+collected so far. 
+
+*Longer REPL files would cause measurably*
+*longer recompilation/testing times.*
+
+Also, longer repl files would be slower to debug when they fail,
+*It is better to have several smaller files vs one huge REPL file.*
+
+### Example :
+```
+a := 1
+println(a)
+===output===
+1