parser: do not allow 1 char struct names

vlib/v/checker/tests/struct_pub_field.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/struct_pub_field.v:9:4: error: field `i` of struct `A` is immutable
+vlib/v/checker/tests/struct_pub_field.v:9:4: error: field `i` of struct `Foo` is immutable
     7 |         i: 1
     8 |     }
     9 |     a.i = 2

vlib/v/checker/tests/struct_pub_field.vv

@@ -1,9 +1,9 @@
-struct A {
+struct Foo {
 	i int
 }
 
 fn main() {
-	a := A{
+	a := Foo{
 		i: 1
 	}
 	a.i = 2

vlib/v/parser/struct.v

@@ -41,6 +41,9 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 	if language == .v && p.mod != 'builtin' && name.len > 0 && !name[0].is_capital() {
 		p.error_with_pos('struct name `$name` must begin with capital letter', end_pos)
 	}
+	if name.len == 1 {
+		p.error_with_pos('struct names must have more than one character', end_pos)
+	}
 	// println('struct decl $name')
 	mut ast_fields := []ast.StructField{}
 	mut fields := []table.Field{}