Convert po commands to the rzshell

[XVilka]

DO NOT SQUASH

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

• Converted all po commands to rzshell without any addition or removal
• Refactored rz_core_transform_op() and rz_core_write_block_op_at() to use size_t instead of int for buffer size (hexlen).

Test plan

CI is green

Closing issues

Partially addresses #1590