Refactor `ModIface` serialisation to allow adding more deduplication tables

Split off from #24540, we propose to refactor the serialisation of ModIface.

The main goal is to change UserData, which has special support for deserialising Names and FastStrings. See:

data UserData =
   UserData {
        -- for *deserialising* only:
        ud_get_name :: BinHandle -> IO Name,
        ud_get_fs   :: BinHandle -> IO FastString,

        -- for *serialising* only:
        ud_put_nonbinding_name :: BinHandle -> Name -> IO (),
        -- ^ serialize a non-binding 'Name' (e.g. a reference to another
        -- binding).
        ud_put_binding_name :: BinHandle -> Name -> IO (),
        -- ^ serialize a binding 'Name' (e.g. the name of an IfaceDecl)
        ud_put_fs   :: BinHandle -> FastString -> IO ()
   }

This does not scale if we want to add more deduplication tables such as #24540. Moreover, a lot of these fields are populated by undef.

This issue proposes to generalise UserData to allow arbitrary deduplication tables with minimal changes to the Binary class interface.

We split UserData (and BinHandle) into Read/Write, allowing us to always keep only exactly the information we need.

The second step is to generalise how putWithTables serialises deduplication tables, allowing us to conveniently add more deduplication tables later.