write: add `Section::{data, data_mut}` accessors (#367)
write: add `Section::{data, data_mut}` accessors (#367)

This commit adds `Section::{data, data_mut}` accessors to
`write::Section` to allow accessing data after it's been appended. The
motivation for this is that in Wasmtime we build an `Object` but we try
to patch relocations after the locations of all symbols are known,
handling them before load-time. Currently the logic of
appending-with-alignment is duplicated in Wasmtime but allowing access
to the underlying section data as-been-appended-so-far should enable us
to remove that duplication.
gimli-rs/object