Shuffling files for Racket R6RS.

* Renamed *.sld -> *.sls
* Moved core os.sls inside the os directory.
* Tweaked library syntax accroding to R6RS.
Fixed compatibility errors.

* Replaced `assert` from (rnrs base) with the one from (os utils assert).

* Replaced `sort` from (srfi :95) with `list-sort` from (rnrs sorting).

* Replaced `filter` from (srfi :1) with the one from (rnrs lists).

* Added (srfi :1) for ternary `member`.

* Added (rnrs lists) for `memq`.

* Added (rnrs control) for `unless` and `when`.

* Replaced |symbol| syntax with (string->symbol "symbol").

* Corrected syntax of (export (rename ...) ...) for R6RS.

* Added dummy expressions at the end of certain library-level `begin`
  forms. This is necessary because R6RS `library` does not explicitly
  support definition splicing via `begin`.

* Corrected a discrepancy in SRFI-69: `hash-table-values` returns
  immutable lists, but R6RS procedures are defined for mutable lists.
  Added (compatibility mlist) and a call to `list->mlist` to fix this.

* Added an extra #f argument to R6RS `error` calls.

* Corrected `define-record-type` syntax for R6RS.

* Removed `cond-expand` usage (not [easily] supported in R6RS).

* Replaced (begin) forms with (begin #f).
Wrote *truth* in the README.
Merge branch 'downgrade-to-r6rs'

I have moved from Chibi R7RS to Racket R6RS because debugging is easier with
DrRacket. Effective changes are minimal, so it will not be hard to move back
to R7RS when everything is a little bit more stable. Just need to avoid any
R6RS-specific things.