Expand necessary builtin macros eagerly

[CohenArthur]

The following list of macros is allowed to expand eagerly as of Rust 1.49. In our current implementation, this will mean tweaking that builtin's handler in rust-macro-builtins.cc in order to return a new fragment containing the unexpanded macro expansion. The list is as follows:

• [x] Implement eager expansion for concat!()
• [ ] #1794
• [x] #1795
• [ ] #1806
• [ ] #1796
• [ ] #1797
• [ ] #1798
• [x] #1804
• [x] #1805
• [x] #1865

I'll create associated issues for each of these tasks.

[CohenArthur]

Updated as most of the macros on this list did not need to be expanded eagerly :)