Fails to build on nightly

[wez]

The build (for ssh2's tests) is failing on nightly rust because of the extprim transitive dep via the unmaintained syntex crate. Can this dependency be replaced?

error: legacy asm! syntax is no longer supported
   --> C:\Users\wez\.cargo\registry\src\github.jparrowsec.cn-1ecc6299db9ec823\extprim-1.7.0\src\u128.rs:884:9
    |
884 |           asm!("
    |           ^---
    |           |
    |  _________help: replace with: `llvm_asm!`
    | |
885 | |             movq $2, %rax
886 | |             mulq $3
887 | |             movq %rax, $0
...   |
891 | |         : "r"(left), "r"(right)
892 | |         : "rax", "rdx");
    | |________________________^

error: aborting due to previous error

error: could not compile `extprim`.

[sagebind]

Here's the extprim PR: kennytm/extprim#18. It hasn't been published yet though: kennytm/extprim#19.