A more flexible table.remove

["Soni \"They/Them\" L." <fakedme@...>]

table.remove has some unnecessary restrictions:

> Removes from list the element at position pos, returning the value of 
the removed element. When pos is an integer between 1 and #list, it 
shifts down the elements list[pos+1], list[pos+2], ···, list[#list] and 
erases element list[#list]; The index pos can also be 0 when #list is 0, 
or #list + 1; in those cases, the function erases the element list[pos].
>
> The default value for pos is #list, so that a call table.remove(l) 
removes the last element of list l.

This should be changed such that:

- pos can be a non-integer. in which case, the value is simply removed.
- pos can be negative.
- basically pos should be called index and it should just shift things 
up-one or down-one when it makes sense to do so.

This allows new code to use table.remove instead of t[x] = nil. This 
would start a transition period between tables-with-holes and nil-in-tables.

table.insert should be changed similarly (for consistency), with one 
small difference - table.insert on non-integer indices should error if 
there's already a value.

Examples:

t = { 1, 2, 3, hello="1", world="2" }

table.remove(t, "hello") -- ok, returns "1"; t is now { 1, 2, 3, world="2" }
table.remove(t, 2) -- ok, returns 2; t is now { 1, 3, world="2" }

pcall(table.insert, t, "world", "3") -- returns nil, "attempt to insert 
into an existing index"

t = { 1, 2, nil, 3, 4, 5, nil, 6, 7, 8 }

table.remove(t, 1) -- ok, returns 1; t is now { 2, nil, nil, 3, 4, 5, 
nil, 6, 7, 8 }
table.insert(t, 5, 9) -- ok, t is now { 2, nil, nil, 3, 9, 4, 5, 6, 7, 8 }