Cleanup strs

[erickt]

This is a cleanup pull request that does:

• removes os::as_c_charp
• moves str::as_buf and str::as_c_str into StrSlice
• converts some functions from StrSlice::as_buf to StrSlice::as_c_str
• renames StrSlice::as_buf to StrSlice::as_imm_buf (and adds StrSlice::as_mut_buf to match vec.rs.
• renames UniqueStr::as_bytes_with_null_consume to UniqueStr::to_bytes
• and other misc cleanups and minor optimizations

[huonw]

This should probably have a warning about maintaining the invariant that the string is valid UTF8, and should definitely take &mut self rather than &self.

[erickt]

Thanks for catching that. I'll add the &mut self and a warning.

[erickt]

@huonw: Updated the pull request address your suggestion, and also added a couple more cleanup functions.

[graydon]

I would think to_bytes should remain to_bytes_with_null or such, no? We named those APIs so they're clear about null termination.

(The whole matter of null termination remains, to this day, contentious. As you know, in #7235)

[erickt]

@graydon: Most of this pull request was extracted from the non-controversial parts of #7172, so my original intention was that there would no longer be strings with nulls in them :) I can rename to_bytes to to_bytes_with_null for now, and continue the debate for my next round of #7172.

[erickt]

@graydon: Updated the pull request to add str.to_bytes_with_null().