[Spec Deviation] Allow any simple-const-expr in describing singleton types

[MaryamZi]

Description:
As per the spec (v0.990:2019-01-16 - page 29, 30),

"singleton-type-descriptor := simple-const-expr

A singleton type is a type containing a single shape. A singleton type is described using an compile-time constant expression for a single value: the type contains the shape of that value. Note that it is possible for the variable-reference within the simple-const-expr to reference a structured value."

simple-const-expr :=
  nil-literal
  | boolean
  | [Sign] int-literal
  | [floating-point-type] [Sign] floating-point-literal
  | string-literal
  | variable-reference
floating-point-type := < (decimal|float) >

Unsupported expressions need to be supported.

Affected Versions:
0.990.2

[MaryamZi]

The updated grammar for simple-const-expr (as of 2020r1) is as follows

simple-const-expr :=
  nil-literal
  | boolean-literal
  | [Sign] int-literal
  | [Sign] floating-point-literal
  | string-literal
  | constant-reference-expr

We need to support the following two expressions;

type D +1;
type F +1.0;

Everything else is supported;

type A ();

type B true;
type C false;

type E -1;

type G -1.0;

type H "str";

type I FOO;

const FOO = "foo";