Add SRFI 101: Purely Functional Random-Access Pairs and Lists

This one is tricky, since it requires the redefinition of
very fundamental things like CAR and QUOTE.

- Everything is defined inside the module with a prefix
  "srfi101:", and exported with renaming
- The ideal way to use this SRFI is to prefix symbols when
  importing, otherwise one'd lose the standard car, list, quote
  etc.
- Its tests are in a separate file, and loaded as a last thing
  in do-test.stk

Issues:

- When not renamed, quote enters a loop
- Tests run fine and pass when called manually, but when inside the
  test system, they raise some kind of exception

SUPPORTED-SRFIS

@@ -71,6 +71,7 @@ implemented in latest version is available at https://stklos.net/srfi.html):
     - SRFI-96: SLIB Prerequisites
     - SRFI-98: Interface to access environment variables
     - SRFI-100: define-lambda-object
+    - SRFI-101: Purely Functional Random-Access Pairs and Lists
     - SRFI-111: Boxes
     - SRFI-112: Environment Inquiry
     - SRFI-113: Sets and Bags