lang: ast, parser, interfaces: Implementation of nested class sugar

This implements a new type of syntactic sugar for the common pattern of
a base class which returns a child class, and so on. Instead of needing
to repeatedly indent the child classes, we can instead prefix them at
the definition site (where created with the class keyword) with the name
of the parent class, followed by a colon, to get the desired embedded
sugar.

For example, instead of writing:

class base() {
	class inner() {
		class deepest() {
		}
	}
}

You can instead write:

class base() {
}
class base:inner() {
}
class base:inner:deepest() {
}

Of course, you can only access any of the inner classes by first
including (with the include keyword) a parent class, and then
subsequently including the inner one.

lang/parser/lexer.nex

@@ -62,6 +62,10 @@
 /:/		{
 			yylex.pos(lval) // our pos
 			lval.str = yylex.Text()
+			// sanity check... these should be the same!
+			if x, y := lval.str, interfaces.ClassSep; x != y {
+				panic(fmt.Sprintf("COLON does not match ClassSep (%s != %s)", x, y))
+			}
 			return COLON
 		}
 /;/		{

lang/parser/parser.y

@@ -257,7 +257,7 @@ stmt:
 		}
 	}
 	// `class name { <prog> }`
-|	CLASS_IDENTIFIER IDENTIFIER OPEN_CURLY prog CLOSE_CURLY
+|	CLASS_IDENTIFIER colon_identifier OPEN_CURLY prog CLOSE_CURLY
 	{
 		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtClass{
@@ -268,7 +268,7 @@ stmt:
 	}
 	// `class name(<arg>) { <prog> }`
 	// `class name(<arg>, <arg>) { <prog> }`
-|	CLASS_IDENTIFIER IDENTIFIER OPEN_PAREN args CLOSE_PAREN OPEN_CURLY prog CLOSE_CURLY
+|	CLASS_IDENTIFIER colon_identifier OPEN_PAREN args CLOSE_PAREN OPEN_CURLY prog CLOSE_CURLY
 	{
 		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtClass{
@@ -1408,7 +1408,7 @@ colon_identifier:
 		posLast(yylex, yyDollar) // our pos
 		$$.str = $1.str
 	}
-	// eg: `foo:bar` (used in `docker:image`)
+	// eg: `foo:bar` (used in `docker:image` or `class base:inner:deeper`)
 |	colon_identifier COLON IDENTIFIER
 	{
 		posLast(yylex, yyDollar) // our pos