Separate `src` into specific namespaces, and move modules to features about darkfi

[narodnik]

Also the old architecture.md document had this hierarchy:

• system/ contains core functionality, types and utilities.
• net/ is the p2p networking base layer.
• zk/ is the zk lisp virtual machine and related zk code.
• crypto/ for the transaction builder and merkle tree classes.
• node/ is a fully working darkfi node.
• wallet/ key generation and management features such as derivation and storage.
• service/ any services such as the cashier or gateways.
• chain/ blockchain database functionality and state transition.
• rpc/ interface for remote procedure calls, includes adapter to interface with the node backend and the default jsonrpc transport mechanism.

[narodnik]

We should keep rpc/ in the main library somewhere since it is very useful to be able to add a jsonrpc server to an app. Maybe under util/rpc/?

service can maybe go in node/service/.

[ghassmo]

i finished this part:

• moved wallet, client, service and state modules to node directory and made it a feature
• made blockchain directory work as a feature
• create zk directory and moved vm module to that
• moved serial and error modules to util directory
• moved darkfid, gatewayd, and drk binaries to sub-crates in bin/ directory
• moved cashierd and all the bridges to bin/cashier sub-crate

[lunar-mining]

as of commit #d001a140dc9ce4c26eaaa02f8e747da2b21c00c2 this is the current directory hierarchy:

/src
    /chain
    /cli
    /crypto
    /net
    /node
        /service
        /wallet
    /system
    /tui
    /tx
    /util
        /rpc
    /zk

/bin
    /cashierd
    /darkfid
    /drk
    /ircd
    /map

there are a few outliers that were not considered in the plans above, in particular:

/src 
    /tui
    /cli
/bin 
    /map

any preferences on what to do with them?