Space-saving header columns

[wasamasa]

The languages could be displayed rotated 45° and the columns be set to have a uniform width.

See https://ecraven.github.io/r7rs-benchmarks/ and https://css-tricks.com/rotated-table-column-headers/ for a demo.

[lassik]

Patches welcome.

On a more general note, there are so many Scheme implementations that the table is getting unwieldy to read. We should think of a convenient way to narrow the matrix down to a few implementations at a time.