MIPS removed by Rust due to rust-lang/rust#115218

lms.yml action/job shall also be triggered by this branch.

See https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

- Last change (replace "set-output" by $GITHUB_OUTPUT) caused an error.
- replace checkout@v2 by checkout@v4 due to
  https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

See https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Replace (as suggested in online discussions) with "dtolnay/rust-toolchain"
due to deprecated node12. See:
https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/
https://github.com/dtolnay/rust-toolchain

- contains just the module structure
- adapt lib.rs

Add "mod helper" to sst/mod.rs to integrate/build/test new file helper.rs.

For all "HssParameter"s, only one top part height for "dividing" one tree
is needed.

Leave it to "sst_demo"-caller to provide different seeds.

- Use "REF_IMPL_MAX_ALLOWED_HSS_LEVELS" instead of magic number for max HSS size.
- Add new call argument for demo (two-step key generation)

Adapt LmsPrivateKey so the restricted number of leaves and
instance number can be considered.

Dep. on signing entity, tree height and division, calc. and
set first and last LMS leaf to limited values.

First set of arguments can optionally have top_height and entity_number.
If one is provided, the other is mandatory.
If not provided, behaviour is as previously (HSS).

- extension not written to file yet
- keys are used as before
- tests are still working

- 2 Bytes used for top_height and signing_entity
- calling with parameters 0/0 for those values
  results in same behaviour as before

- "node value" is the hash value calc. via get_tree_element()
  depending in the root node of the signing instance's single-sub-tree

- adapt AUX data: add level for signing entity nodes; ignore levels "above"
- generate the correct VerifyingKey: replace our own ID 1 "pubkey" value with node value calc. via intermediate SI nodes
- use separate files for different signing entities numbers (aux, prv, pub)
- extend arguments for demo code

- improve and clean up example code and API calls to lib code
- add adapted HssPublicKey "from()" method for SST to adapt AUX data,
- store other nodes intermediate data in AUX data
- use public key generated via signing entity intermediate nodes in HssPublicKey
- fix sst helper functions

- Generate keys for several signing entities (via parameter)

- provide number of signing instances instead of "top_div_height"
- provide SSTS parameter separately

- partially populate AUX data in gen key step 1
- read/use and extend AUX data in gen key step 2
- fix AUX data (finalize, write HMAC)
- adapt AUX arguments for py script and sst_demo
-

- fix for "tree identifier": the value needs to be the same for all signing entities
  - create tree ident. via first signing entity, then use this tree ident. for other entities
  - here the temp. solution is a fixed value for the treee ident.
- fix for "used leafs": the position of the used lefas value in the u64
  "CompressedUsedLeafsIndexes" and in the HSS levels array was wrong if several HSS levels were used

- build strings via f-string (improve readability)
- format code
- add requirements.txt (so far empty) to communicate python environment

- rename some constants, variables and functions to reflect their purpose
- remove obsolete comments
- remove unused functions
- format via "cargo fmt"

Simplification is possible because AUX data is filled before
calculating the "LmsPublicKey", so the "real" public key
can now be calculated straight via "LmsPublicKey" with index 1.

Before we used the LmsPublicKey with sst extension and index != 1 which
would result in the intermediate node value.

Use common tree identifier (RFC: "I") so that the key generation and signing
all use the same value (as opposed to using different values dep. on their
different seeds).

- rename "top_div_height" -> "l0_top_div"
- comments to indicate TODOs, review, questions...

All files under " res/nrf52-arm/".
See res/nrf52-arm/README.md for instructions.