Check-in [ba804fbdae]

This TIP proposes a new `[array default]` subcommand that allows default values
for arrays, causing that value to be returned any time an attempt is made to
access an element of the array that isn't present.

# Rationale

For its most basic usages, a Tcl array variable can be seen as a collection of
scalar variables. However, an array also has its own existence as a variable and
provides specficic actions on itself and on its elements. For exampe, the
regular way to check for a variable existence is to use the `[info exists]`
command, and this is true for both arrays and scalars (including array
elements); likewise, the `[set]` command is used to read and write the value of
a scalar vars and array elements, however array variables have no value in
themselves but provide a way to get and set several of its elements at once.

For more advanced usages, one must get past the simple 'collection of scalars'
viewpoint. A typical example is sparse arrays: at present, an implementation
requires a combination of `[array exists]` (or `[info exists]`) and `[set]`: the
former to test the presence of a key in an array, the latter to get or set the
actual value, and in this order. Failure to do so is likely to cause runtime
errors.

This document therefore proposes to add a new `[array default]` subcommand in
order to support such advanced usages. Sparse arrays built on this new feature
will be simplerw, more robust, faster, all the while being memory efficient.

# Specification

A new `array default` subcommand will be created with the following syntax:

> `array default ` _`option arrayName ?arg?`_ : Default value management. 
   
The following _option_ values are supported:

* <code>array default get <em>arrayName</em></code>: Get _arrayName_
  default value. Raise an error if _arrayName_ is not an array variable or if it
  has no default value.
* <code>array default set <em>arrayName</em> <em>value</em></code>: Set
  _arrayName_ default value. If the variable _arrayName_ does not already exist,
  it is created with an empty array value. Raise an error if _arrayName_ is an
  existing scalar variable or an invalid array name.
* <code>array default exists <em>arrayName</em></code>: Test whether
  _arrayName_ has a default value. Return false if _arrayName_ does not exist.
  Raise an error if _arrayName_ is not an array variable.
* <code>array default unset <em>arrayName</em></code>: Unset _arrayName_ default
  value. Does nothing if _arrayName_ does not exist. Raise an error if
  _arrayName_ is not an array variable.

# Examples

Initial state:

	% array exists var
	0
	% array default exists var
	0
	% array default get var
	"var" isn't an array
	% set var(a)
	can't read "var(a)": no such variable

Create array with one entry:

	% set var(a) 1
	1
	% array exists var
	1
	% array default exists var
	0

Attempt to read missing entry:

	% set var(b)
	can't read "var(b)": no such element in array

Add default value:

	% array default set var 0
	% array default exists var
	1
	% array default get var
	0

Default value is only used by missing entries:

	% info exists var(a)
	1
	% set var(a)
	1
	% info exists var(b)
	0
	% set var(b)
	0

You can `lappend` to a missing entry and it will behave as expected:

	% lappend var(b) 2
	0 2
	% info exists var(b)
	1

Changing the default value only impact missing entries:

	% info exists var(c)
	0
	% set var(c)
	0
	% array default set var 9999
	% array default get var
	9999
	% set var(a)
	1
	% set var(b)
	0 2
	% set var(c)
	9999

Default values don't show in entry lists or iterations:

	% array names var
	a b
	% array get var
	a 1 b {0 2}
	% array for {key value} var {puts "$key => $value"}
	a => 1
	b => 0 2
	% parray var
	var(a) = 1
	var(b) = 0 2	

Unsetting the default value will only invalidate missing entries:

	% array default unset var
	% set var(a)
	1
	% set var(b)
	0 2
	% info exists var(c)
	0
	% set var(c)
	can't read "var(c)": no such element in array

Setting the default value on a non-existing variable implicitly creates an array
variable:

	% array exists var2
	0
	% array default set var2 1234
	% array exists var2
	1
		

# Implementation

The proposed implementation is available on branch
[tip-508](https://core.tcl.tk/tcl/timeline?r=tip-508) in the Tcl Fossil
repository.