Add support for nan:canonical and nan:arithmetic in assert_return

[abrown]

Now that WebAssembly/spec#1104 is merged and the new syntax is appearing in spec tests, I am hoping to add support for nan:canonical and nan:arithmetic to assert_return in wast. Here's what I think is necessary:

• deprecate WastDirective::AssertReturn[.+] with #[deprecated(since = "5.1.0", note = "Please use AssertReturn instead")]; I think it makes sense to leave these around until all of the spec tests are converted over and users of wast update their spec tests
• modify Lexer::number to lex nan:arithmetic to a new variant FloatVal::NanArithmetic and nan:canonical to a new variant FloatVal::NanCanonical (I was thinking of just lexing to FloatVal::Nan and setting val but I'm not sure this works for different sizes)
• in float!, implement the variants FloatVal::NanArithmetic and FloatVal::NanCanonical; note that this means that Float32 and Float64 will be able to be parsed from nan:canonical and nan:arithmetic everywhere (not just in assert_return statements) which I don't think is technically correct but might actually be useful
• bump to 5.1.0; I think this should be a minor version bump because no existing functionality is removed