Support prof.dump for jemalloc-ctl.

[yuhao-su]

I'm trying to implement prof.dump for jemalloc but I met some problems that can be generally applied to all functions that take a string argument.

The write interface usually take a &'static str as an input argument, however, the raw::write_str take a C-style string the end with \0. I can think of two solutions:

1. Take ffi::CStr as input. Let users pass a C-style string.
2. include alloc in jemalloc-ctl and convert a Rust-style "`&'static str" to a C-stype string.

Could you let me what is the best practice for this? Thanks!

[BusyJay]

CStr should be used.

[yuhao-su]

Okay, I'll submit a pr later