[RUNTIME][IR] String operator+

[tqchen]

We have a few places in the code base that uses operator+ of String. Right now:

• we either convert to std string https://github.com/apache/incubator-tvm/blob/master/src/tir/ir/expr.cc#L86
• Or added some overload that converts to std::string https://github.com/apache/incubator-tvm/blob/master/include/tvm/runtime/container.h#L1391

Given that operator+ is quite common, and we can have a better solution in these cases(by allocating the right result length then copy), perhaps we should have a good overload for most cases.

[tqchen]

cc @zhiics @wweic @junrushao1994

[junrushao]

I agree, not sure about the performance impact though

[tqchen]

k, it is mainly about the usability, than the perf aspect

[zhiics]

Besides this, I think we should make the comparison operators free functions instead of member functions because they are not communicative.

[tqchen]

@zhiics would you be interested in taking a stab?

[zhiics]

#5806

[tqchen]

Closing thanks to @zhiics !