Functions for retrieving the AssetId and Amount

[Hrom131]

Describe the feature

Simplicityhl-std should include functions for easily retrieving the AssetId and Amount for inputs and outputs.

Simplicityhl-std should provide the IndexType enum:

enum IndexType {
    Input,
    Output,
}

Functions for retrieving an AssetId:

1. get_explicit_asset_id(index: u32, index_type: IndexType)
2. get_confidential_asset_id(index: u32, index_type: IndexType)
3. get_explicit_input_asset_id(index: u32)
4. get_confidential_input_asset_id(index: u32)
5. get_explicit_output_asset_id(index: u32)
6. get_confidential_output_asset_id(index: u32)
7. get_current_explicit_asset_id()
8. get_current_confidential_asset_id()

Functions for getting Amount:

1. get_explicit_amount(index: u32, index_type: IndexType)
2. get_confidential_amount(index: u32, index_type: IndexType)
3. get_explicit_input_amount(index: u32)
4. get_confidential_input_amount(index: u32)
5. get_explicit_output_amount(index: u32)
6. get_confidential_output_amount(index: u32)
7. get_current_explicit_amount()
8. get_current_confidential_amount()

Functions for retrieving the AssetId along with the Amount:

1. get_explicit_asset_id_with_amount(index: u32, index_type: IndexType)
2. get_confidential_asset_id_with_amount(index: u32, index_type: IndexType)
3. get_explicit_input_asset_id_with_amount(index: u32)
4. get_confidential_input_asset_id_with_amount(index: u32)
5. get_explicit_output_asset_id_with_amount(index: u32)
6. get_confidential_output_asset_id_with_amount(index: u32)
7. get_current_explicit_asset_id_with_amount()
8. get_current_confidential_asset_id_with_amount()