Commands generator should not rely on arrays generator

[dubzzz]

Fixes #192

Before this development fc.commands was relying on fc.array to build and shrink commands correctly.
Unfortunately, with this setup it was not possible to benefit from assumptions we can make on commands while shrinking them:

• the last started command should always be kept because it is the one causing the failure (or none of the commands caused the failure)
• string representation was featuring dash (-) for non executed commands
• string representation was not reliable when commands were used along with other shrinkable arbitraries

[coveralls]

Coverage decreased (-0.7%) to 91.734% when pulling 5210427 on feat/commands into 81e8adc on master.