scmutil: add a simple key-value file helper The purpose of the added class is to serve purposes like save files of shelve or state files of shelve, rebase and histedit. Keys of these files can be alphanumeric and start with letters, while values must not contain newlines. In light of Mercurial's reluctancy to use Python's json module, this tries to provide a reasonable alternative for a non-nested named data. Comparing to current approach of storing state in plain text files, where semantic meaning of lines of text is only determined by their oreder, simple key-value file allows for reordering lines and thus helps handle optional values. Initial use-case I see for this is obs-shelve's shelve files. Later we can possibly migrate state files to this approach. The test is in a new file beause I did not figure out where to put it within existing test suite. If you give me a better idea, I will gladly follow it.