support explicit specification of the optic target

[aplavin]

The main common usecase is using @(re)set with an array element, as in @set $(x[i]).a.b = 123 and @reset $(x[i]).a.b = 123.
Currently, these require a temporary variable:

old = x[i]
new = @set old.a.b = 123

and

old = x[i]
x[i] = @set old.a.b = 123

[aplavin]

@jw3126 thanks for raising those concerns with @reset and potentially surprising mutability. Indeed, avoiding mutations unless explicitly asked for is best.

Still, from time to time I wish there was this functionality for @set, would make stuff more convenient. So, in the new commit $... works in @set, but not @reset.

Also, the same commit fixes the rare but nasty @reset bug unrelated to $:

julia> @reset last(x, n) = [1, 2, 3]
last (generic function with 1 method)

- it redefined the function!

[aplavin]

Bump @jw3126 :) Would be nice to have this feature, now that there's no mutation anywhere (as a sideeffect, this fixes the reset bug mentioned above).
Typical usecases look like this:

@set f($x, 5).a = 3
@insert f(5, $x).a[2] = 3
@set f($(xs[i]).a) = 3
...

Now, these require explicit set(@optic ) call or a temporary variable.

[jw3126]

Agreed, this is nice.