Preserve case sensitive column names

[Symbolics]

Column names are represented by symbols in DATA-FRAME. While this is convenient for several reasons, like symbol plists for column properties, it has the effect of converting all column names to upper case. This means that most of the time you cannot 'round-trip' exactly to/from a DATA-FRAME. The problem typically becomes an issue when exchanging data with other systems, for example Vega-Lite, that may be case sensitive.

There are a few possible solutions:

• Preserve the original case in the symbol plist.
• Use a formatting convention to allow converting between format. CFFI does this with their translate-* functions.
• Adjust the readtable-case to :preserve during the import.