fix(no-std): added spinlock for config checking

[mkatychev]

using ecies-rs in a no_std context produced compilation errors complaining about the use of std in parking_lot.

The parking_lot crate is not no_std compliant whereas lock_api is, added a spinlock backed lock_api compliant RwLock for the configuration using the spin crate when std feature is disabled.

[kigawas]

@mkatychev Thanks for your contribution. Can you try #140 to check whether it's working for you?

[mkatychev]

Hi @kigawas I'm sure the code works but I will not review AI generated output unfortunately.

[kigawas]

0.2.10 is now released