optional simulate & wallet, editable TransactionBuilder

- Can now pass an `account` OR `wallet` when constructing the
  ContractClient, or none! If you pass none, you can still make view
  calls, since they don't need a signer. You will need to pass a
  `wallet` when calling things that need it, like `signAndSend`.

- You can now pass `simulate: false` when first creating your
  transaction to skip simulation. You can then modify the transaction
  using the TransactionBuilder at `tx.raw` before manually calling
  `simulate`. Example:

      const tx = await myContract.myMethod(
        { args: 'for', my: 'method', ... },
        { simulate: false }
      );
      tx.raw.addMemo(Memo.text('Nice memo, friend!'))
      await tx.simulate();

- Error types are now collected under `AssembledTransaction.Errors` and
  `SentTransaction.Errors`.

src/contract_spec.ts

@@ -719,7 +719,7 @@ export class ContractSpec {
         args: Record<string, any>,
         options: MethodOptions
       ) => {
-        return await AssembledTransaction.fromSimulation({
+        return await AssembledTransaction.build({
           method: name,
           args: spec.funcArgsToScVals(name, args),
           ...options,