Generate keys from Fortuna rather than always using the OsRng

When creating a Secp256k1, we attach a Fortuna CSRNG seeded from the
OS RNG, rather than using the OS RNG all the time. This moves the
potential RNG failure to the creation of the object, rather than at
every single place that keys are generated. It also reduces trust
in the operating system RNG.

This does mean that Secp256k1::new() now returns an IoResult while
the generate_* methods no longer return Results, so this is a breaking
change.

Also add a benchmark for key generation. On my system I get:

test tests::generate_compressed   ... bench:    492990 ns/iter (+/- 27981)
test tests::generate_uncompressed ... bench:    495148 ns/iter (+/- 29829)

Contrast the numbers with OsRng:

test tests::generate_compressed   ... bench:     66691 ns/iter (+/- 3640)
test tests::generate_uncompressed ... bench:     67148 ns/iter (+/- 3806)

Not too shabby :)

[breaking-change]