Attached is a patch that adds a new (web json) module. Some may
remember that I submitted a patch back in 2015 (time flies, eh?) for
an (ice-9 json) module that never made it in. Well, 10 years is a long
time and Guile still doesn't have a built-in JSON module. Third party
libraries like guile-json and guile-sjson are available, the latter
being an adaptation of my original patch and the former remaining the
go-to library used by larger Guile projects like Guix. There's also
SRFI-180 (which sounds like a cool surfing trick!) which was published
in 2020 but the API is, in my opinion, overly complicated due to
generators and other things.  Anyway, JSON continues to be *the* data
interchange format of the web and Guile really ought to have a simple
API that can read/write JSON to/from a port using only Scheme data
types that have read syntax (i.e. no hash tables like guile-json).
This minimal, practical API is what my patch provides.  I've tried my
best to make it as efficient as possible.

I've settled on the following JSON<->Scheme data type mapping which is
nearly identical to SRFI-180 with the exception of object keys:

- true and false are #t and #f
- null is the symbol 'null
- numbers are either exact integers (fixnums and bignums) or inexact
reals (flonums, NaNs and infinities excluded)
- strings are strings
- arrays are vectors
- objects are association lists with string keys (SRFI-180 chose
symbols but JSON uses strings so strings feel the most honest)

Thanks in advance for the review,

- Dave