Split off Subtensor of Unbroadcast into its own rewrite

It seems to add a non-trivial 100ns

Signed-off-by: Emmanuel Ferdman <emmanuelferdman@gmail.com>

Also add checks for runtime broadcast

These show up in the gradient of Convolve1D

Such that its name can be more informative.

They could get tested independently.
This was mostly done to understand their purpose.

...in print statements

Convert lines when `parse=False` from generator to list.
The result when `parse=False` is only used in a warning message,
and I don't think the generator would even print properly.

That's the only possible definition, and the type checker is freaking out.

The backslashes in `__file__` created invalid escape sequences.