Misc conversion optimizations/fixes for protocol 21

[dmkozh]

This is an issue to track several small issues found and recorded in https://github.com/stellar/rs-soroban-env/pull/1314/files, such as:

• Symbol::try_from_val has to use only a single conversion function based on length
• symbol_new_from_slice should use SCSymbol::try_from_bytes instead of duplicating its logic with slight modification
• map_new_from_linear_memory should use Symbol::try_from_val on the slice instead of doing XDR roundtrip
• deserialize_from_bytes should use to_host_val directly