Add \ overload handling calling a Nil def

[kurtlawrence]

This syntax would be useful for patterns such as append \+ $i.foo $i.bar.
That is, where the + is usually typed and will not work on an input such as TableRow.
Typically, the pattern is append { let {get bar} $b | get foo | + $b }.

Generalising this is tricky.

Implementation option

The \<cmd> arg1 arg2 ... argN is sugar for an intrinsic, which will amount to let $i | \#n | <cmd> arg1 arg2 ... argN.
This benefits from having <cmd> be whatever is already defined, along with doing the let $i portion.
The drawback is cmds (such as +) would now need a Nil input, which is going to place further burden on the inferencer.

• Bikeshed $i
• Update documentation -- provide an example and explanation #112