prost-build: optimise derived prost::Name

[mina86]

When deriving prost::Name, prost-bulid generates the following code
for full_name and type_url methods:

fn full_name() -> ::prost::alloc::string::String {
    ::prost::alloc::format!(
        "some.package.{0}", Self::NAME
    )
}
fn type_url() -> ::prost::alloc::string::String {
    ::prost::alloc::format!(
        "example.com/{0}", Self::full_name()
    )
}

With those definitions, type_url:

• allocates a new temporary string,
• formats text into that string,
• allocates another string,
• and once again formats text into that string.

Most of those operations can be done at build time with only a single
string allocation being necessary in type_url method.

Change the generated code such that no formatting happens at run time
and no temporary strings are allocated in type_url, specifically:

fn full_name() -> ::prost::alloc::string::String {
    "some.package.MessageName".into()
}
fn type_url() -> ::prost::alloc::string::String {
    "example.com/some.package.MessageName"
}

Furthermore, unconditionally derive type_url even if no domain is
specified such that default definition (which uses temporary strings
and formatting) isn’t used.

[caspermeijn]

The codebase already has tests for this functionality.

LGTM

[mina86]

error: package half v2.3.1 cannot be built because it requires rustc 1.70 or newer, while the currently active rustc version is 1.65.0

What about this failure then?

[caspermeijn]

error: package half v2.3.1 cannot be built because it requires rustc 1.70 or newer, while the currently active rustc version is 1.65.0

What about this failure then?

It seems the MSRV is no longer accurate. This PR is trying to fix that: #975

[caspermeijn]

Please rebase to the latest master branch to fix CI

[caspermeijn]

This code doesn't change the behavior and already has tests.