Replace usage of static keyword with const

[genbattle]

This RFC was merged in recently, and today when I went to build nanovg I found that it wouldn't build with the following error:

/home/nick/.cargo/git/checkouts/nanovg-rs-84b890ac0c756c09/master/src/lib.rs:69:9: 69:15 error: no rules expected the token `static`
/home/nick/.cargo/git/checkouts/nanovg-rs-84b890ac0c756c09/master/src/lib.rs:69         static LEFT         = ffi::NVG_ALIGN_LEFT,
                                                                                        ^~~~~~
Could not compile `nanovg`.

Judging from the RFC, the solution is to replace this particular usage of static with const.

[KevinKelley]

Weird that Travis has been passing... thanks for the heads-up! --changed 'static' to 'const' in a few places, builds okay now. Let me know if any problems.

[genbattle]

Seems to have been a pretty recent change, just because I updated today on the nightly. Many other packages seem to be broken with this issue, but I'm glad I could help out at least a little bit!