Support non-struct types

[philhofer]

Things get rather difficult if we don't want to generate code for arbitrary types, but still want to support something like:

type Chunk [128]byte

type Block struct {
    Meta string
    Data Chunk
}

Right now, Chunk is interpreted as IDENT, even though we could conceivably generate the appropriate code for it in-situ. However, generating the appropriate methods for Chunk is simpler than transitively applying its type information across every location in which it is used.

TL;DR generate code for all valid *ast.TypeDecl nodes.

[philhofer]

See #60

[philhofer]

Fixed by #60