Add better handling of Seralization Errors internally

[volfco]

Right now, ZkStruct can blow up if the struct changes; like so:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error("missing field `resources`", line: 0, column: 0)', /home/colum/.cargo/registry/src/github.com-1ecc6299db9ec823/zkstate-0.1.6/src/lib.rs:249:43
stack backtrace:
   0: rust_begin_unwind
             at /rustc/b3e19a221e63dcffdef87e12eadf1f36a8b90295/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /rustc/b3e19a221e63dcffdef87e12eadf1f36a8b90295/library/core/src/panicking.rs:92:14
   2: core::option::expect_none_failed
             at /rustc/b3e19a221e63dcffdef87e12eadf1f36a8b90295/library/core/src/option.rs:1329:5
   3: core::result::Result<T,E>::unwrap
             at /home/colum/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/result.rs:1037:23
   4: zkstate::state_change
             at /home/colum/.cargo/registry/src/github.com-1ecc6299db9ec823/zkstate-0.1.6/src/lib.rs:249:17
   5: zkstate::ZkState<T>::initialize
             at /home/colum/.cargo/registry/src/github.com-1ecc6299db9ec823/zkstate-0.1.6/src/lib.rs:90:9
   6: zkstate::ZkState<T>::new
             at /home/colum/.cargo/registry/src/github.com-1ecc6299db9ec823/zkstate-0.1.6/src/lib.rs:71:9
   7: stardance::executor::Executor::new
             at ./stardance/src/executor.rs:43:19
   8: executor::main
             at ./stardance/examples/executor.rs:42:20
   9: core::ops::function::FnOnce::call_once
             at /home/colum/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

There should be:

1. better handling of this error so it doesn't panic
2. documentation around best practices on changing the fields of this object