Add ability to "saturate" inlining

[saulshanabrook]

This PR extends the inline_leaves function added in #4 to support repeatedly calling it.

It does this by adding a check when constructing the new operator string. If the operator string was already inlined, we replace all instances of · (the missing arg constant), with the new values. Otherwise, do the previous behavior of constructing a calling signature.

It also adds a saturate_inline_leaves function, that repeatedly inlines until it is a no-op.

I have added this as an example to include in the readme. E.g. this is the physics_scientific_unsolved_4h_ellisk_2019 inlined once:

Compared to the saturated inline:

Motivation

When you use a wrapper struct around a literal you might want this inlined as well, to make the graph have less overlapping edges. So I wanted to be able to run inline_leaves at least twice.

I had been wanting to try the saturated version too for a while to see if it helps with readability at all.

[saulshanabrook]

Ping on this PR. I am currently using it in the Python bindings