[integer] Remove multiply_toom_cook_3 for BigUInt

[forfudan]

This pull request includes significant changes to the src/decimojo/biguint/arithmetics.mojo and src/decimojo/biguint/biguint.mojo files, focusing on the removal of the multiply_toom_cook_3 function, a correction to the shift_words_left function's documentation, and the addition of a new method to the BigUInt struct.

Major Changes:

Removal of multiply_toom_cook_3 function:

• The multiply_toom_cook_3 function, which implemented the Toom-Cook 3-way multiplication algorithm, has been entirely removed. This includes all associated logic and comments.

Documentation Correction:

• The documentation for the shift_words_left function in arithmetics.mojo was corrected to state that the function adds trailing zeros, not leading zeros.

New Method in BigUInt struct:

• A new method shift_words_left has been added to the BigUInt struct, which shifts the words of the BigUInt to the left by a specified number of bits. This method calls the shift_words_left function in arithmetics.mojo for its implementation.