Change cache key to tuple and read/write list of dictionaries to json

[jcbrill]

Change msvc cache key from string to tuple.

Convert cache dictionary to a list of dictionaries with key tuple converted to key list when writing cache json file.
Convert list of dictionaries with key list converted to key tuple to the cache dictionary when reading cache json file.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation

[jcbrill]

Feel free to ignore.

Per request:

Is the tuple-ification of the key planned?
@jcbrill could you share the changes If my count is correct, it would take approximately two new lines of code and changes to three lines of code. ?

@mwichmann Do you see anything of concern?

Note: there is an opportunity to increase the efficacy of the cache by normalizing the script path and case on Windows as discussed in SCons#4111.

[mwichmann]

This seems fine, I'm inclined to merge this change. Maybe a comment why we're doing what we're doing with came out of the json file (and/or what goes into it).

[jcbrill]

Better or worse? See below as well.