use ToString for formatting in Evaluation

[mmatera]

In the same direction as PR #1376, this PR starts to decouple formatting routines from the mathics core.

• evaluation.format implements formatting by applying ToString[HoldForm[], format] to the result.
• doctest now parses strings in a way that we can use "[charname]" inside the expected results.

The idea is that, at the end, every conversion be consistent with the output of ToString.

[TiagoCavalcante]

@mmatera what is the status of this?

[mmatera]

This was also a try to reorganize the way in which formatting is done. The whole question was If the code for formatting (spread into Evaluation.evaluate, BaseExpression.format, BaseExpresion.do_format, and BaseExpression.boxes_to_... ) and the code in ToString should be shared, or if they are different things. I think now we have taken a different direction, so this is outdated