Simplify cover variable naming

[mourner]

A simpler and cleaner way to generate a hashed variable for cover stats — we take first 12 hex digits of sha1 digest and convert it to base36. I think the first 12 digits are more than enough to avoid any possible collisions — we could probably do away with less. cc @bcoe

// before
++__cov_bpZamYSu4YZDor3bpoXygHqmoUk.s[0];

// after
++cov_173mm3ogj2.s[0];

[yahoocla]

CLA is valid!