Allow dynamic nested types

[Swoorup]

It would be nice to get support for HashMap, for eg.

#[derive(Debug, PartialEq, StructOfArray)]
#[soa_derive(Debug, PartialEq)]
struct Baz {
    foo: u16,
    bar: u8,
    test: HashMap<u16, u8>,
}

would create

struct BazVec {
    #[doc = r" a vector of `"]
    #[doc = stringify!(foo)]
    #[doc = r"` from a"]
    #[doc = "[`Baz`](struct.Baz.html)"]
    pub foo: Vec<u16>,
    #[doc = r" a vector of `"]
    #[doc = stringify!(bar)]
    #[doc = r"` from a"]
    #[doc = "[`Baz`](struct.Baz.html)"]
    pub bar: Vec<u8>,
    #[doc = r" a vector of `"]
    #[doc = stringify!(test)]
    #[doc = r"` from a"]
    #[doc = "[`Baz`](struct.Baz.html)"]
    pub test: HashMap<u16, Vec<u8>>,
}

instead of the current

struct BazVec {
    #[doc = r" a vector of `"]
    #[doc = stringify!(foo)]
    #[doc = r"` from a"]
    #[doc = "[`Baz`](struct.Baz.html)"]
    pub foo: Vec<u16>,
    #[doc = r" a vector of `"]
    #[doc = stringify!(bar)]
    #[doc = r"` from a"]
    #[doc = "[`Baz`](struct.Baz.html)"]
    pub bar: Vec<u8>,
    #[doc = r" a vector of `"]
    #[doc = stringify!(test)]
    #[doc = r"` from a"]
    #[doc = "[`Baz`](struct.Baz.html)"]
    pub test: Vec<HashMap<u16, u8>>,
}