idea: EnvString task input type to implicitly define environment variable

[mlin]

Declarations in a task (excluding outputs) allow a special type EnvString which is String in all respects, but furthermore, implicitly defines a shell environment variable in the task container, with the corresponding name and value.

[patmagee]

@mlin I would be interested in seeing this as an idea in WDL :)

[mlin]

superseded by #617