lang: Implementation of tuples and tuple structs in IDL

[cherryman]

Since JavaScript arrays are heterogenous, they're the canonical representation of tuples in JS. TypeScript also supports typing them, so for a homogenous array of T you might do T[], but tuples are instead of the form [T, U, V]. So a rust type Struct(U, V) maps to [U, V]. This can be represented in the IDL by adding another "variant" called "tuple", and the associated "fields" being a list of IDL types. This also creates an elegant translation of rust coproduct types, so we map

enum Enum {
    VariantOne(T, U, V),
    VaraintTwo { x: T },
 }

to

type Enum = { variantOne: [T, U, V] } | { variantTwo: { x: T } }

IDL Example

"type": {
  "kind": "tuple",
  "fields": [
    "u8",
    "publicKey",
    { "array": ["i64", 20] },
    { "defined": "CustomType" }
  ]
}

Related to #232.

[dovahcrow]

This will come very handy for upgrading the PDAs.
Instead of having

#[account]
pub struct PDA {
    field1: u64,
    _pad: [u8; 32],
}

Where _pad is for future upgrades. You can now have

#[account]
pub enum PDA {
    V1 {
        field1: u64,
    },
    Pad([u8; 32])
}

so that you can easily add a new variant to the enum for account upgrade, as long as the size of the whole account does not exceeds the size of Pad.