- Add test to verify piped string replace exit code
Ensure fields parsing error messages are the same.
Note: C++ relied upon the value of the parsed value even when `errno` was set,
that is defined behaviour we should not rely on, and cannot easilt be replicated from Rust.
Therefore the Rust version will change the following error behaviour from:
```shell
> string split --fields=a "" abc
string split: Invalid fields value 'a'
> string split --fields=1a "" abc
string split: 1a: invalid integer
```
To:
```shell
> string split --fields=a "" abc
string split: a: invalid integer
> string split --fields=1a "" abc
string split: 1a: invalid integer
```
This implements the primary environment stack, and other environments such
as the null and snapshot environments, in Rust. These are used to implement
the push and pop from block scoped commands such as `for` and `begin`, and
also function calls.
