fix: complete the implementaion of `stringconcat`

[silver-ymz]

The implementation of stringconcat hasn't been completed. It will raise error execute error: conversion error: no function ||(Utf8, Utf8), which needs to add concat function in binary_op.

Also, could I ask the reason of O::Item: Sized limit in binary_op? Could I remove it?

[skyzh]

Yes of course we can remove it. I guess it was introduced to make it easier to implement some functions in the past.

[wangrunji0408]

Stringconcat was added to the planner casually and was never fully supported. Thanks for your efforts to make it work!