Add element number to Buffer and MutableBuffer

[largeword]

Description

Add number of contained elements to Buffer and MutableBuffer data types.

Motivation and context

This provides a convenient way to get the total length of the buffer, which can be helpful for data transfer to and from external devices.

How has this been tested?

Pass compilation for ghc-9.2.5.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.