Fix memory leak in setting encodings

	There is a memory leak in Encoding.default_external= and
	Encoding.default_internal= because the duplicated name is not freed
	when overwriting.

	    10.times do
	      1_000_000.times do
	        Encoding.default_internal = nil
	      end

	      puts `ps -o rss= -p #{$$}`
	    end

	Before:

	     25664
	     41504
	     57360
	     73232
	     89168
	    105056
	    120944
	    136816
	    152720
	    168576

	After:

	    9648
	    9648
	    9648
	    9680
	    9680
	    9680
	    9680
	    9680
	    9680
	    9680

now `enc_table->list` is not a pointer.

This reduces global lock acquiring for reading.
https://bugs.ruby-lang.org/issues/18949

* And simplify callers of get_actual_encoding().
* See [Feature #18949].
* See https://github.com/ruby/ruby/pull/6322#issuecomment-1242758474

* See [Feature #18949].

[Misc #18891]

Otherwise it's way too easy to confuse it with US_ASCII.

Main doc for encodings moved from encoding.c to doc/encodings.rdoc

Less macros == huge win.