Docs and links

+# Here I collect some interesting links.
+
+## cargo environment
+
+Currently I always build all dependencies within th projects own .cargo subdir
+by setting the environment variable CARGO_HOME.
+
+    export CARGO_HOME=./.cargo
+
+Actually I am quite unsure if that is really a good idea because this results
+in a lot of rebuilds for each new project.
+
+## Implement Monads with rust.
+
+Which is currently difficult through impossible, but might be feasible with
+some small additions to the rust type system, namely
+«(generic) associated traits.»
+
+[Read here](https://varkor.github.io/blog/2019/03/28/idiomatic-monads-in-rust.html)
+
+### Is impl Trait what we needed?
+
+A new language feature which sounded a bit like the needed one for above is:
+
+[impl Trait](https://medium.com/@iopguy/impl-trait-in-rust-explanation-efde0d94946a)
+
+Crossreading it a bit seems to indicate that this is not like the above needed
+language feature.
+
+## WASM ... on gentoo ...
+
+Right now rust is not really a first class citizen within the gentoo eco
+system. For that reason at least when compiling rust with the `system-llvm`
+use flag some manual preparations are needed.
+
+### rustwasm tutorial
+
+[rustwasm](https://rustwasm.github.io/book/introduction.html)
+
+### location of rustlib ...
+
+`wasm-pack` expects the wasm runtime under /usr/lib/rustlib/ but a gentoo
+installation puts it under /usr/lib/rust-1.39.0/rustlib/. To come around this
+issue I just created a symlink.
+
+### Missing lld
+
+Again `wasm-pack` searches for llvm-lld or rust-lld for the link phase. This
+is currently not a dependency for llvm with WASM support, so I installed it
+manually. Finally you have to tell rustc to use lld as linker via this
+environment variable:
+
+    export RUSTFLAGS="-C linker=lld"