Create a new module 'fastexcel-base'/'fastexcel-common'

[meiMingle]

To share some data structures between writer and reader, such as DynamicBitMatrix, DynamicByteArray, BitMatrix.
The longer-term goal is to achieve efficient reading of merged cells. @ochedru DDo you have any advice on this, friend?

[ochedru]

Hi @meiMingle, sounds like a great idea to me! Unfortunately I do not have much time to look into it, and that's why you are now a maintainer 😊