port to meson cargo

Use the new unstable meson cargo support. This simplifies the
meson.build script and allows to use crates such as clap that require
picking up features from Cargo.toml.

This also allows us to not embed thiserror in subprojects, and instead
use a wrap file with a custom meson.build and some patches to make it
compile without running its build.rs script.

.docker/Dockerfile

@@ -6,7 +6,7 @@ RUN getuto
 
 RUN emerge-webrsync
 
-RUN emerge --jobs=$(nproc) \
+RUN emerge \
     =dev-lang/rust-bin-9999 \
     llvm-core/clang \
     llvm-core/lld \