use elements.rs as chemistry database

[moritz-gross]

while wandering some more through the codebase, I saw that we have a whole periodic table laying around here.
I think this is something we'd better hand off.
I found the neat crate elements_rs for this. In particular, having categories like ElementCategory::NobleGas (not to speak of AlkalineEarthMetal etc.)

next to refactoring code, I this also uncovered a buggy test re Ammonia: The electronegativity of Nitrogen is 3.04, and that of Hydrogen is 2.20, so NH_3 is in descending order.

most likely, there is way more to hand off in this file, but this looks like an easy-to-do thing for now.

[moritz-gross]

TODO: check difference in MathCAT size, and how to judge such a thing

[moritz-gross]

binary size seems to actually shrink ?!?.

Running this:

git switch main
cargo clean
cargo build --release --lib --locked
cp target/release/liblibmathcat.so /tmp/mathcat-main.so

git switch use-elements-rs-as-chemistry-database
cargo clean
cargo build --release --lib --locked
cp target/release/liblibmathcat.so /tmp/mathcat-elements.so

stat -c '%n: %s bytes' \
    /tmp/mathcat-main.so \
    /tmp/mathcat-elements.so

gives the result

/tmp/mathcat-main.so: 605344 bytes
/tmp/mathcat-elements.so: 604152 bytes

The "eliminating-dead-code" step you mentioned is Link-Time Optimization as I just learned?! Seems to work well then.

@NSoiffer have you ever used the size command for inspecting section sizes of binaries? That could be interesting to inspect it further, but I think this would be more of a fun side-project than actually pragmatic here.

So in summary, this PR seems good to go