Update cexpr dependency

[kpcyrd]

This bumps nom from 0.3 to nom 0.4 internally. Note that 3 tests started to fail due to that change:

failures:

---- header_issue_674_1_hpp stdout ----
thread 'header_issue_674_1_hpp' panicked at 'libclang error; possible causes include:
- Invalid flag syntax
- Unrecognized flags
- Invalid flag arguments
- File I/O errors
If you encounter an error missing from this list, please file an issue or a PR!', libcore/option.rs:989:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.

---- header_issue_674_2_hpp stdout ----
thread 'header_issue_674_2_hpp' panicked at 'libclang error; possible causes include:
- Invalid flag syntax
- Unrecognized flags
- Invalid flag arguments
- File I/O errors
If you encounter an error missing from this list, please file an issue or a PR!', libcore/option.rs:989:5

---- header_issue_674_3_hpp stdout ----
thread 'header_issue_674_3_hpp' panicked at 'libclang error; possible causes include:
- Invalid flag syntax
- Unrecognized flags
- Invalid flag arguments
- File I/O errors
If you encounter an error missing from this list, please file an issue or a PR!', libcore/option.rs:989:5


failures:
    header_issue_674_1_hpp
    header_issue_674_2_hpp
    header_issue_674_3_hpp

test result: FAILED. 425 passed; 3 failed; 0 ignored; 0 measured; 0 filtered out

The other dependencies that need updating are:

• clang-sys
• proc-macro2
• quote
• which

[emilio]

Hmm, do we have any idea of why the regressions? They don't make much sense to me, there's no chance that we've ran cexpr so early in the process I think...

[emilio]

Ok, so I looked at the regression, it's a nom or shlex regression. But it's test-only, fwiw. So I'll update the tests and get this landed.