add statistics module

provides some basic statistics functions (sum, mean, etc.)
and a function to return the parameters of the linear regression of
two variables

implemented using num_traits to be more flexible for the types

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

Cargo.toml

@@ -30,6 +30,7 @@ lazy_static = "1.4"
 libc = "0.2"
 log = "0.4"
 nix = "0.16"
+num-traits = "0.2"
 once_cell = "1.3.1"
 openssl = "0.10"
 pam = "0.7"