nt_array_ptr design can require source changes for initialization by calls.

[dtarditi]

@nmeum has reported that the nt_array_ptr design where only a 0 can be written at the upper bound doesn't interact well with function calls that do initialization, such as memset. See #242 for details. Given initialization code of the form:

char *p = ...
memset(p, 0, len)

If p is changed to an nt_array_ptr they had to narrow the bounds by 1 and insert an explicit initialization step:

memset(p, 0, len - 1);
p[len] = `\0`;

memset currently takes an array_ptr. In Issue #242, I discuss two possible approaches that we could take to avoid these source code changes:

• treat functions that initialize by 0 specially for nt_array_ptr.
• significantly expand our vocabulary for talking about restrictions on writes via array_ptr. This would be challenging to do.