Use prod_or_fast!

[liamaharon]

I'm using the prod_or_fast! macro a lot in #1708 to keep the new config clean. This follows convention in runtimes maintained by Parity and is less verbose.

It allows replacing code like this:

#[cfg(not(feature = "fast-blocks"))]
pub const INITIAL_SUBNET_TEMPO: u16 = 360;

#[cfg(feature = "fast-blocks")]
pub const INITIAL_SUBNET_TEMPO: u16 = 10;

#[cfg(not(feature = "fast-blocks"))]
pub const INITIAL_CHILDKEY_TAKE_RATELIMIT: u64 = 216000; // 30 days at 12 seconds per block

#[cfg(feature = "fast-blocks")]
pub const INITIAL_CHILDKEY_TAKE_RATELIMIT: u64 = 5;

with this

pub const INITIAL_SUBNET_TEMPO: u16 = prod_or_fast!(360, 10);

// 30 days at 12 seconds per block = 216000
pub const INITIAL_CHILDKEY_TAKE_RATELIMIT: u64 = prod_or_fast!(216000, 5);

[basfroman]

LGTM from Cortex related parts

[open-junius]

It is a really good macro.