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.

compiler/optimization.v

@@ -1,40 +0,0 @@
-module main
-
-// `a in [1,2,3]` => `a == 1 || a == 2 || a == 3`
-// avoid allocation
-// `typ` is the type of `a`
-// `ph` is for string_eq()
-fn (p mut Parser) in_optimization(typ string, ph int) {
-	p.check(.lsbr)
-	mut i := 0
-	// Get `a` expr value (can be a string literal, not a variable)
-	expr := p.cgen.cur_line.right(ph)
-	is_str := typ == 'string'
-	//println('!! $p.expr_var.name => $name ($typ)')
-	for p.tok != .rsbr && p.tok != .eof {
-		if i > 0 {
-			if is_str {
-				p.gen(' || string_eq($expr, ')
-			}	else {
-				p.gen(' || $expr == ')
-			}
-		}	
-		if i == 0 {
-			if is_str {
-				p.cgen.set_placeholder(ph, ' string_eq(')
-				p.gen(', ')
-			} else {
-				p.gen(' ==')
-			}	
-		}	
-		p.check_types(p.bool_expression(), typ)
-		if is_str {
-			p.gen(')')
-		}	
-		if p.tok != .rsbr {
-			p.check(.comma)
-		}
-		i++
-	}	
-	p.check(.rsbr)
-}