Implement low_level_call into the multisig

Type of change

The following changes have been made:

transfer and execute_transaction have been merged into a single function: execute_transaction
SRC-2 has been implemented
A single hash function compute_hash has replaced the separate hash functions.
The use of arbitrary bytes in hash functions has been removed, more detail available in notes below.
pub(crate)

Arbitrary bytes in hash functions have been removed as they served to add additional entropy; however this is not necessary as knowing the hash does not create a vulnerability, the hash must still be signed over by a multisig user.
The test functions::info::compute_hash::gets_call_hash has been written, but is ignored with the following reason:

Encoding of nested structs with Bytes fields in the RustSDK doesn't match the encoding in Sway. This is not an issue for the use of compute_hash when making a contract call; as the user would not be creating the hash in the SDK

Closes #22 Closes #717 Closes #718

Implementing SRC-2 Closes #424