Implement symbol_index_in_linear_memory

[bbyalcinkaya]

This PR introduces the symbol_index_in_linear_memory host function. It performs a linear search to find the index of a given symbol in an array of byte slices.

This function is used in converting SCVal values to enums by locating symbols that match enum variants. The test_custom_types contract is added to define a sample enum type and test conversions between Val and enum representations.

[ACassimiro]

Looks good to me.