Introduce enum syntax

[patmagee]

A long standing request from users has been the ability to define enum constants where a user can restrict the values an input can contain and easily refer to those values in a named way. While restricting the values is accomplishable with validation (see #657), being able to easily refer to those constants is another matter.

At the risk of making WDL a little bit more like a programming language, I propose we add a new construct called enum

enum Foo {
  FOO
  BAR
  BIZ
}


workflow my_workflow {
  input {
    Foo val
  }

  if (Foo.BAR == val) {
    call bar
  }

}

Semantics:

• Input values are provided as JSON strings, where the String value is expected to equal the value of the enum
• The strings are case sensitive ("Foo" != FOO)
• A String can be converted to an enum Foo foo = "FOO"

[jdidion]

Duplicate of #139?

[patmagee]

Huh, yup. This is what happens when you do not look at the history @jdidion

[patmagee]

closing in favour of #139