Export Signer to WASM

[SupremoUGH]

Goes together with Manta-Network/sdk#78.

Changes:

• Refactored most SignerState methods as free functions.
• Included sign_with_transaction_data compound method with a test.
• Removed std from wallet feature because of compatibility issues with sdk. Changed the AssetMap from HashMap to BTreeMap as a consequence.
• Several trait implementations (mostly (de)Serialize and Hash) for compatibility with sdk.
• Several type definitions.
• Removed network feature.

Misc:

• Removed unused utxo_utilities module.
• Improved TransactionData verification method.

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[zqhxuyuan]

Mabye rename package name methods to impl or util, or functions?
PS: I prefer functions instead of methods. like substrate way: https://github.com/paritytech/substrate/blob/master/frame/assets/src/functions.rs