Switch from TryFrom to FromStr for Manifest

[xfbs]

This PR changes Manifest and RawManifest to implement the FromStr trait rather than the TryFrom<String> trait.

While both traits can be used and do achieve a similar purpose, there is one important difference in their semantics related to what they do with the string they are taking:

• The TryFrom<String> attempts to consume a String and convert it into the target type (Manifest or RawManifest). The ownership of the String is thereby moved during the conversion. This is therefore useful if you do need ownership of the input type.
• The FromStr trait attempts to parse a &str (reference to a string) into the target type (Manifest or RawManifest). It does not take ownership of the string, only a read-only reference to it. This is therefore useful if you only need to read the input string.

Since fundamentally, are are just parsing a string and we do not need it anymore after the parsing operation, the FromStr trait is more useful here because it does not require us to consume to the string. The TryFrom<String> trait would be more appropriate if we wanted to hold on to the original string after the parsing operation, and perhaps store that inside the Manifest type.

But since we simply discard the string, there is no good reason to use TryFrom. For this reason, this PR changes the code base to use FromStr instead.

[mara-schulke]

Thank you 😊 good catch!

[xfbs]

Good point! I do prefer parse() as well. I changed all invocations of from_str() to parse().