This is a rather large PR that got out of hand 😅 But it fixes and
improves quite a few things.

There is a lot of parsing going on left and right by various parts:
reading the files, the external test tools, and parsing of what they
output. Previously it was *really* hard to grok what was parsing what
and what exactly failed.

It took me ages to grok why these tests were failing for the TOML
library; turns out this was a mixture of actual bugs in the TOML
library, bugs in the toml-test-* tools, bugs in this test tool, and
tests people had added that were just broken.

This vastly redoes the test output to look quite nice and understandable
(see below), it's also a lot more verbose, but that's a feature IMO.
Instead of just "expected type string, got int" actually show what we
have. There is some room for improvement here as it shows this with
fmt.Printf("%#v"), which gives rather Go-specific output. Maybe show as
JSON? Have to see later.

Some other changes as well:

- All JSON files are formatted consistently according to what
  json.MarshalIndent(.., "", "  ") outputs. Being predictable means it's
  much easier to compare the desired and actual outputs since you don't
  have to mentally parse the differences in formatting so much.

- Fix a bunch of broken tests, and arrays can now be represented as just
  "[ ... ]"; no object with type/value needed (or even supported). It
  was added in case "TOML might support tuples", but we'll cross that
  bridge if we ever get at it.

  Fixes #52
  Fixes #58
  Fixes #67

- Tests are now consistently referenced with the full path relative to
  the test directory: "valid/name" and "invalid/name". e.g. to run one
  test use "toml-test [..] valid/name"

  This means it's clearer what kind of test we're dealing with, means we
  don't need globally unique tests, and that we can add "js-valid" tests
  specifically for a language if need be.

  All filesystem access now also done trough the fs.FS interface; the
  big upshot of this is that we can produce binaries with the tests
  embedded in them and that you don't need to pass -testdir (this still
  works, for development etc.)

  The downside is that it requires Go 1.16, but I think that's okay for
  a tool like this. We can just provide binaries once a new release is
  tagged.

  Fixes #20
  Fixes #23

----

Example output (test name and some other parts are in bold unless you
add -no-bold):

	Test: invalid/key-multiline  (toml-test-decoder < ../toml-test/tests/invalid/key-multiline.toml)
	    Expected an error, but no error was reported.

	    input sent to toml-test-decoder:
		"""long
		key""" = 1

	    output from toml-test-decoder (stdout):
		{
		  "long\nkey": {
		    "type": "integer",
		    "value": "1"
		  }
		}

	    want:
		<empty>

	Test: valid/inline-table-nest  (toml-test-encoder < ../toml-test/tests/valid/inline-table-nest.json)
	    Error encoding TOML: toml: TOML array element cannot contain a table

	    input sent to toml-test-encoder:
		{
		  "arr_arr_tbl_empty": [
		    [
		      {}
		    ]
		  ],

		  [.. trim ..]

	    output from toml-test-encoder (stderr):
		Error encoding TOML: toml: TOML array element cannot contain a table

	    want:
		<empty>