Remove the need for allocation from is_utf8_string.

[hovinen]

Previously, IsEncodedStringMatcher converted its actual value input to a Vec and constructed a String out of that in order to match it. This is because using a string slice caused problems with the borrow checker as described in #323.

The fundamental problem is that the type against which the inner matcher is matching is a reference whose lifetime must be named in the trait bound:

impl<ActualT: AsRef<[u8]> + Debug, InnerMatcherT> Matcher
    for IsEncodedStringMatcher<ActualT, InnerMatcherT>
where
    InnerMatcherT: Matcher<ActualT = &'what str>,
                                       ^^^^ What goes here???

One option is just to remove the reference:

impl<ActualT: AsRef<[u8]> + Debug, InnerMatcherT> Matcher
    for IsEncodedStringMatcher<ActualT, InnerMatcherT>
where
    InnerMatcherT: Matcher<ActualT = str>,

This doesn't work when the inner matcher is eq, since one would then be comparing an str and a string slice:

verify_that!("A string".as_bytes(), is_utf8_string(eq("A string"))) // Compile error: cannot compare str and &str

However, this problem does not occur with StrMatcher:

verify_that!("A string".as_bytes(), is_utf8_string(constains_substring("A string"))) // Passes

So this also introduces an easy way to obtain a StrMatcher to check string equality:

verify_that!("A string".as_bytes(), is_utf8_string(eq_str("A string"))) // Passes

This is slightly inconvenient, since one must use a different matcher to compare string equality in this case. But it is well-documented and not too inconvenient to use. So it should be okay.

The introduction of eq_str also allows fixing a longstanding limitation of the matcher property! -- that it could not match on properties returning string slices. This PR therefore also updates the documentation accordingly and adds an appropriate test.

[gribozavr]

@hovinen Thank you for the PR! This generally makes sense to me, however the PR has developed merge conflicts with the code at HEAD. Would you mind rebasing? Please reassign back to me when done.