Clarification of Slice, Vector and Array

Especially in the tutorial beginners should not be confused with
wrong terminology. It helps to know the right names for things
when you want to find something in the documentation.
Implement Mutable trait for StrBuf.
auto merge of #14102 : moonglum/rust/slice-clarification-in-readme, r=kballard

There are no arrays in Rust, they are slices. Especially in the tutorial beginners should not be confused with wrong terminology. It helps to know the right names for things when you want to find something in the documentation.

@erickt explained that today to me and it helped me a lot when getting started :wink: Maybe we should also explain what a slice and what a vector is in the tutorial. If you like that, I will try to do that and attach that to the pull request :wink:
auto merge of #14110 : SSheldon/rust/strbuf_mutable, r=alexcrichton

Despite implementing the Container trait, StrBuf did not implement the Mutable trait and had no clear method; this request implements the clear method of the Mutable trait for StrBuf.

Testing done: added a test and ran `make check` without any failures.