Add no_std compatibility

[febo]

Problem

Currently the solana-security-txt is dependent of the std library. When writing no_std programs, it creates a conflict due to the differences in how the panic handler behaves when the std is present:

error[E0152]: found duplicate lang item `panic_impl`
  --> program/src/entrypoint.rs:20:1
   |
20 | pinocchio::nostd_panic_handler!();
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: the lang item is first defined in crate `std` (which `solana_security_txt` depends on)

Solution

Remove the std dependency. This is achieved by:

• Replacinfg the use of std::collections::HashMap by hashbrown::HashMap
• Bump dependencies to use no_std compatible versions
• Add alloc as an external crate
• Use Vec and String types from alloc
• Fix clippy warnings: upercase static variable name (SECURITY_TXT) and implement TryFrom<&str> for Contact

These changes have no impact on the logic of the macro – the result is the same: