Change target folder to /target and move on to version 0.13

[webmaster128]

Closes #89

[maurolacy]

LGTM.

[chipshort]

LGTM

[webmaster128]

I tested this with the expected results on a local workspace with 9 contracts. In all cases, no local ./target folder was created. I.e. the optimizer does not mess with the local build directory.

No target cache set (not recommended but still solid)

• Run 1: 5:58,49 total (the first one created the registry_cache cache. The following ones are a minute faster)
• Run 2: 4:55,18 total
• Run 3: 5:00,33 total
• Run 4: 5:19,70 total

Target mounted (--mount type=volume,source="$(basename "$(pwd)")_cache",target=/target)

• Run 1: 5:29,11 total
• Run 2: 27,760 total (huge win here, as expected)
• Run 3: 28,324 total
• Run 4: 29,328 total