Masked email: the fork's subspace and the masked_email module (ME-1, ME-2, ME-3, ME-6a, ME-7, ME-8, ME-13, ME-14, ME-15)

Subspace _ holds the fork's own data, with its own SQL table and RocksDB
column family, and is part of backup. The masked_email module keeps each
mask's state, last mail and pending deadline beside upstream's record, an
index from address to mask with tombstones, and a per-account change log;
and generates addresses in the fork's format.

crates/features/src/lib.rs

@@ -18,4 +18,5 @@
 //! it. It works on registry objects and the store directly, never on
 //! `common::Server`.
 
+pub mod masked_email;
 pub mod tenancy;