Feature: Add support for env variable namespacing

[jazzu]

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Using same structure definition and environment variables for separate, concurrent routines was impossible. The need occurs, when the same service is used with different configurations within one program.

For example, you might have an internal microservice, which other routines within your program uses. Each would need their own configuration and credentials to define different accesses on the same resource. These cannot co-exist as environment variables without namespacing them.

This feature gives the user the possibility of defining multiple variables with same root but different prefixes and have them available through instances of the same structure.

Includes documentation, tests, examples.