[stdlib] Change os.env functions to use StringSlice

[martinvuyk]

Change os.env functions to use StringSlice

[owenhilyard]

env args are mutable, so it's probably best that either the runtime makes a copy at the start of the program or that we do make a copy here due to hazards around C libraries modifying them.

[soraros]

@owenhilyard The setenv (C version makes a internal copy unlike putenv) and unsetenv take a StringSlice in Mojo land, and the getenv performs a copy on return. I don't see any issues with this approach.

[soraros]

Actually, do we want to remove unsafe_cstr_ptr (different from unsafe_c_char_ptr which is a purely ABI thing)? We would lose an important API contract, namely, null termination, even if only from a documentation perspective.

[martinvuyk]

Actually, do we want to remove unsafe_cstr_ptr (different from unsafe_c_char_ptr which is a purely ABI thing)? We would lose an important API contract, namely, null termination, even if only from a documentation perspective.

I've wanted to deprecate unsafe_cstr_ptr for a long time (remember #3638 ?)

[owenhilyard]

@owenhilyard The setenv (C version makes a internal copy unlike putenv) and unsetenv take a StringSlice in Mojo land, and the getenv performs a copy on return. I don't see any issues with this approach.

That's probably fine so long as we provide locking on the Mojo side, since all of the C library functions are thread unsafe.