add `add` and `sub` to `TaggedLen`

[alejandro-vaz]

add addition and subtraction methods to TaggedLen by generalizing increment/decrement

replace all usages in the codebase of the pattern "get length value, add/subtract and operate meanwhile, then set length to that value by creating a new TaggedLen instance" to directly use these methods

[iamjaeholee]

@alejandro-vaz Hi, I'd like to work on this! Could you assign it to me?

[alejandro-vaz]

yes, it's yours