introduce @as builtin for type coercion

This commit also hooks up type coercion (previously called implicit
casting) into the result location mechanism, and additionally hooks up
variable declarations, maintaining the property that:

    var a: T = b;

is semantically equivalent to:

    var a = @as(T, b);

See #1757

src/codegen.cpp

@@ -8070,6 +8070,7 @@ static void define_builtin_fns(CodeGen *g) {
     create_builtin_fn(g, BuiltinFnIdFrameType, "Frame", 1);
     create_builtin_fn(g, BuiltinFnIdFrameAddress, "frameAddress", 0);
     create_builtin_fn(g, BuiltinFnIdFrameSize, "frameSize", 1);
+    create_builtin_fn(g, BuiltinFnIdAs, "as", 2);
 }
 
 static const char *bool_to_str(bool b) {