String: add from_utf8

[newAM]

Closes #378

Leverages the error type from core::str::from_utf8, but it takes a heapless::Vec as an argument similar to core::String::from_utf8 to avoid allocation errors.

I figured we may as well add the unsafe version too for completeness.

[Dirbaio]

if this is going to copy the bytes anyway, wouldn't it be better to take &[u8]? this way the source of bytes can be something other than a Vec.

(the std method takes a Vec because it can convert the allocatino from Vec to String, without copying)

[newAM]

If it takes bytes: &[u8] then it can run into an allocation error when bytes.len() > N.

Several options:

1. Leave it as-is, for converting from &[u8] the user calls Vec::from_slice followed by String::from_utf8, with each of those having a single error type.
2. Add an error enum with two variants for Utf8Error and bytes.len() > N.
3. Map the Utf8Error onto () to simplify the error type, users can leverage external methods for handling UTF-8 errors.

It's sadly not very ergonomic with any option, I prefer 1, but 2/3 (or another option) would be fine by me. What do you think?

[Dirbaio]

ah true I didn't think about capacity errors. Sounds good to me then. We can always add from_utf8_slice in the future.