Building: Template structs vs builder

Considering two methods of building values to write to buffers.

Template structs:
```rust
data::write(&mut buffer, Foo::Template {
bar: Some(1),
baz: Some("foo"),
sub: Some(FooTemplate {
bar: Some(2),
baz: Some("bar"),
sub: None,
})
});
```

Or builders:
```rust
Foo::Builder::new(&mut buffer)
.set_bar(1)
.set_baz("foo")
.build_sub() // returns Option builder
.some()
.set_bar(1)
.build_baz()
.push("foo")
.push("bar")
.done()
.build_sub()
.none() // returns ()
.build()
```

Values must be written to a buffer in traversal order, so buffers either have to panic if fields are provided out of order (gross), or order of initialization has to be moved into the type system.

Template structs seem nicer and more concise.