Removed string helpers from consumer client

[simlay]

I added these string helpers in #706 because I want to be lazy. @nicholastmosher made some fair points about why they shouldn't be part of the fluvio client.

They're breaking changes so it should go out with the 0.5.0 client.

Also, this should be the last time we do some "quick fixes to the client UI", we should move to some design/review process.

[nicholastmosher]

Thanks for removing these, if it builds it LGTM

[sehz]

rebase

[sehz]

failed in k8-test running locally:

FLUVIO_CMD=true ./target/debug/flv-test --spu 1 --produce-iteration 1000 --develop --client-log warn --server-log debug --skip-checks
Start running fluvio test runner
using k8 environment driver
Executing> /home/ubuntu/fluvio/target/debug/fluvio cluster delete
installing cluster
Executing> /home/ubuntu/fluvio/target/debug/fluvio cluster start --spu 1 --develop --rust-log debug --skip-checks
Waiting up to 60 seconds for Fluvio cluster version check...
Error: 
   0: Failed to install Fluvio on Kubernetes
   1: Timed out when waiting for SPU
panic panicked at 'fluvio cluster start should work: CommandError { command: "/home/ubuntu/fluvio/target/debug/fluvio cluster start --spu 1 --develop --rust-log debug --skip-checks", source: ExitError(1, Output { status: ExitStatus(ExitStatus(256)), stdout: "", stderr: "" }) }', tests/runner/src/environment/k8.rs:67:14
Makefile:78: recipe for target 'smoke-test-k8' failed

[simlay]

We can revisit these changes in a later date. Strings are currently the most common use case for fluvio and this will decrease the amount of boilerplate they need to use to get started.