Support character set, ranges and classes in wildcard modifiers

[xiaq]

In elvish, wildcards can be modified using the indexing operator []. Right now there is only an all wildcard modifier that makes wildcards match dots in the beginning of filenames. For instance, echo *[all] lists all files in the current directory, including those whose names start with a dot.

Another potential candidate for modifier is the traditional character set and range specification, as in echo [aeiou], which matches files whose names consist of a single vowel, and echo [a-z], which matches file whose names consist of a single lower case letter (let's pretend diacritics do not exist). However, square brackets are already used for list and map literals in elvish. Instead we could implement this as a wildcard modifier: echo ?[aeiou]. This gives character ranges more power: now you can also write echo *[aeiou] to match files whose names consist of any number of vowels.

However, the proposal above conflicts with our current modifier all: if it were to be implemented, *[all] will stand for files whose names consist of letters a and l. (We could make a special case for all, since you shouldn't write two l's, but I detest this kind of special casing.) We need to find a syntax to accommodate both our current "match all" functionality and the character set and range functionality.

There is also "character class" in traditional wildcards that look like [[:digit:]], that we also want to include. We can make all a special character class and therefore share its syntax.

For the choice of syntax, we propose the following possibilities:

1. Mimic POSIX, sets and ranges being "bare" and classes needing additional square brackets. Character set is [aeiou], range is [a-z], and class is [[:digit:]]. They may be combined: [[:digit:] aeiou a-z] (the space between aeiou and a-z is not mandatory). Elvish is different in that it requires a leading ?, * or **, of course.

2. Make classes "bare" and make sets and ranges "special classes", one possible syntax being: class is [digit], set is [set:aeiou], and range is [range:a-z]. They may be combined: [digit set:aeiou range:x-z].

[xiaq]

Let's settle on the second choice, I think it is both cleaner and more extensible. We are already quite un-POSIX anyway.