Support default values for fields

[adamconnelly]

Thrift allows fields to be initialised with default values, for example:

struct User {
  1: i32 Id
  2: UserType Type = UserType.User
  3: bool IsEnabled = true
}

There are a number of different types of constant values we need to support:

• Integers (= 123).
• Doubles (= 1.2 or = 1.2e5).
• Strings (= "hello").
• Enum references (= UserType.User).
• Constant references (= DEFAULT_VALUE).
• Lists (= [1, 2, 3]).
• Maps (= { "key": "value", "key2": "value2").

Notes:

• We can't support constant references until Generate a const #8 is implemented.
• We'll need to calculate the type of the constant values, and check they match the type of the field they're being assigned to.