The impetus was a bug that we ran into when writing a string-literal to a fixed-width char field:
```c++
flyweight.putFixedChar("hello");
```
This is unsafe:
- If the field size is less than 6, we overrun the buffer and corrupt it.
- If the field size is more than 6, we don't zero pad the rest of it.
Instead, we build on support for the std::string_view getters and setters, which do length checking.
std::span generalizes this to fixed-width fields of all types. Notably, if the size of the std::span
is knowable at compile time, we pay no runtime cost for the length checking, and we should get
similar performance to the existing API which takes a raw pointer.
Further, we add a sbetool option to disable accepting arrays by raw pointer, which should prevent
memcpy operation without bounds checking. This is off by default to avoid a breaking change.