You'll have to add braces or string() in someplaces that used them.
See test source change.

defaults to fail to conserve old behaviour

Aka: how did this ever work...

with 3.8 we were seeing kinda weird constructs, i.e. in for loops

this is an API breaking change as an additional param is added to
`ValidationInterface::msg` and the ostream inserter for validation
objects necessarily removed

This reverts commit 4ab8d60642141a4b8d495747b932f7d7d9cc495a.

This reverts commit 015c7c76d4b98c0b991c062eb47493ade2f94bae.

This partially reverts 4ab8d60.

The first change is required to allow for
```
Configuration(std::string key, bool value)
```
calls, and the second one is required for
```
Configuration(std::vector<std::string> keys, std::vector<long int> values)
```

[common.configuration] drop obsolete ctor