Ticket 508: Library syntax should have been a subset of R6RS

Per Bothner writes:

I would have preferred to use the R6RS library keywords, and just standardize a subset on the R6RS functionality, even at the risk of some edge-case incompatibility.

Without syntax-case macros, we'd have no include or include-ci, and without the latter, we'd have no clean compatibility with existing R5RS case-insensitive code. Switching from keywords plus code to just keywords (one of which allows embedding code) made such extensibility straightforward.