Make batches atomic

[fewkz]

We could make batches atomic, so that none of the changes are committed until the end of the batch. Any errors that occur during a batch will roll back the changes. This would make errors that occur during a batch more predictable, because currently it will put the derived cells in an invalid state, since the batch that's responsible for updating derived cells failed.

local c = cell(0)
pcall(batch, function()
    c.value = 1
    error("An error occured!")
end)
print(c.value) -- Will print 0, since there was an error during the batch operation.

Batches should be able to be nested and each nested batch should be atomic as well.

local c = cell(0)
batch(function()
    pcall(batch, function()
        c.value = 1
        error("An error occured!")
    end)
    print(c.value) -- Will print 0, since there was an error during the batch operation
end

We should consider making it so reading a value inside of a batch should give whatever the value last was set to in that batch. However this isn't strictly necessary, and might just make it confusing when trying to see why the value of a cell seemingly got reverted.

local c = cell(0)
pcall(batch, function()
    c.value = 1
    print(c.value) -- Will print 1
    error("An error occured!")
end)
print(c.value) -- Will print 0, since there was an error during the batch operation.