Support unnamed structs in IDL

[fanatid]

In continue of #605.

How ⚓ should support unnamed structs (syn::Feilds::Unnamed)? I see two ways:

1. Add arrays support to IDL.
2. Add a new derive macro (AnchorType for example) which will add fields names to such structs:

#[derive(AnchorType)]
struct Z(#[field(name = x)] u64, #[field(name = y)] u64);

Additional derive macro looks better for me because it adds verbosity to JS and makes things more clear.

[armaniferrante]

Arrays are currently supported in the IDL. Is there anything in the current implementation lacking for your use case? For example, generic lengths can't be used at the moment.

[fanatid]

I did not know that arrays are supported. Looks like I need to check IDL again 😄

[archseer]

It would be great if transparent newtype structs were at least supported, for example:

#[derive(
    Clone, Copy, Default, PartialOrd, Ord, PartialEq, Eq, AnchorSerialize, AnchorDeserialize,
)]
#[repr(transparent)]
pub struct Address(pub [u8; 20]);

This compiles fine and generates an IDL with Address references, but the Address type itself is missing. anchor test will then run and report "success! 0 test cases" when it fails to parse the IDL 😄

[acheroncrypto]

Added in #2824.