Speed up json.el encoding

Speed up json.el encoding

This replaces most json-encode-* functions with similar
json--print-* counterparts that insert into the current buffer
instead of returning a string (bug#46761).

Some unused but useful json-encode-* functions are kept for backward
compatibility and as a public API, and the rest are deprecated.

* etc/NEWS: Announce obsoletions.

* lisp/json.el: Document organization of library.  Make subsection
headings more consistent.
(json--encoding-current-indentation): Rename...
(json--print-indentation-prefix): ...to this, to reflect new use.
(json--encode-stringlike, json--encode-alist): Rename...
(json--print-stringlike, json--print-alist): ...to these,
respectively, and encode argument into current buffer instead.  All
callers changed.

(json--print-string, json--print-unordered-map, json--print-array)
(json--print): New functions.
(json-encode-string, json-encode-plist, json-encode-array)
(json-encode): Use them, respectively.

(json-encode-number, json-encode-hash-table): Mark as obsolete
aliases of json-encode.
(json-encode-key, json-encode-list): Mark as obsolete in preference
for json-encode.

(json--print-indentation-depth, json--print-keyval-separator): New
variables.
(json--with-output-to-string): New macro.
(json--print-indentation, json--print-keyword, json--print-key)
(json--print-pair, json--print-map, json--print-list): New
functions.

(json--with-indentation): Use json--print-indentation-depth to avoid
unnecessary string allocation.
(json-encoding-default-indentation, json-pretty-print-max-secs):
Clarify docstrings.
(json--escape, json--long-string-threshold, json--string-buffer):
Remove; no longer used.

* lisp/progmodes/js.el (js--js-encode-value): Replace
json-encode-string and json-encode-number with json-encode.
(js-eval-defun): Use json--print-list to avoid
json-encode-list->insert roundtrip.

* test/lisp/json-tests.el (test-json-encode-number)
(test-json-encode-hash-table, test-json-encode-hash-table-pretty)
(test-json-encode-hash-table-lisp-style)
(test-json-encode-hash-table-sort,  test-json-encode-list):  Replace
uses of obsolete functions with the equivalent use of json-encode.
(test-json-encode-key): Suppress obsoletion warnings.
(test-json-encode-string): Check that text properties are stripped.