compile_i18n as a build function

[ghost]

• Add pub fn compile_i18n in gettext_utils
• Change i18n!, i18n_domain! and include_i18n! so that they use the new paths
• Make i18n! report new strings
• Remove init_i18n! and compile_i18n!
• Update doc
• Test with workspaces
• rerun build.rs when i18n! adds new strings
• remove unused msgids from the pot file
• do not rerun twice after pot is initialized

The way this works now is:

po
├── domain1
│   ├── ar.po
│   ├── de.po
│   └── domain1.pot
└── domain2
    ├── en.po
    ├── pt.po
    └── domain2.pot
target
└── translations
    └── .domains
        ├── domain1
        └── domain2

compile_i18n is called from build.rs. It

• creates po/{domain} and the `.po inside
• creates target/translations/.domains/{domain} (previously target/debug/gettext_macros/{domain})
  It doesn't contain the domain on the first line anymore.
• Set the environment variable GETTEXT_MACROS_DOMAIN to {domain}, it's then used by i18n! and include_i18n!

i18n! now

• rewrites the .pot file if GETTEXT_POT_INIT is not set, then set GETTEXT_POT_INIT
• adds the msgid to the .pot like before

include_i18n! now

• merges the .pot with each .po file for the domain (it fetches the list of locales from target/translations/.domains/{domain}
• compiles each .po file and put the output into the tokenstream directly (without writing to a .mo file, see note)

Removed feature uniform_paths (since it's in stable now), and added proc_macro_diagnostic to allow i18n! to print warnings when new strings are added (don't need it anymore since a second cargo build is not needed).

About the include_i18n not writing to a mo file, this is because otherwise include_bytes! would add an implicit cargo:rerun-if-changed=file.mo, and cargo build would rebuild the whole project every time even if nothing changed.

[ghost]

Still testing the "rerun" mechanism that makes the translations recompile when there's new strings

[ghost]

Looks like it works now

[ghost]

... or not? unused msgids are not removed from the pot.

[ghost]

I moved the pot -> po -> mo compilation logic into include_i18n!, and only one cargo build is needed to have everything at once now...

... BUT it reruns the compiler even if nothing in the project has changed!
:) definitely not a bug