refactor: merged sources of mail-headers,mail-internals,mail-core, mail

Originally it was palaned to do a merge with `--allow-unrelated-history`
but this can not be doesn as `mail-core` has a "invalid" history which
has a merge conflict **with itself**. So even rewinding the history on
a empty repo is not possible.

Instead the code was directly coppied over losing history.

But the history is still available in the different
`history-backup-*` branches. It is just that the past history
is decoupled from the current history.

1 files changed, 8 insertions, 0 deletions

diff --git a/headers/test.sh b/headers/test.sh
new file mode 100755
index 0000000..aa05ef4
--- /dev/null
+++ b/headers/test.sh
@@ -0,0 +1,8 @@
+#!/usr/bin/bash
+
+set -eu
+
+cargo test --all-features "$@"
+cargo clean -p mail-headers
+cargo build "$@"
+rustdoc --test README.md -L./target/debug/deps
\ No newline at end of file