[libcast.lua] Align and optimize the implementations of post-hooks in terms of how they invoke CastCustom()

[dsidirop]

• Place the check GetSpellCooldown() of "CastSpell" inside CastCustom(). All three post-hook functions have to be aligned to use the new CastCustom() which will accept 3 arguments: (id, bookType, rawSpellName)

• Remove the 'for i=1,120 do ...' completely from "CastSpellByName" considering that it's not really serving us right (it doesn't really detect macro-based casts) and it's incredibly taxing on resources

• We can now simplify:

for custom, func in pairs(libcast.customcast) do [...]

Into a simple table-lookup:

local func = libcast.customcast[strlower(rawSpellName)]  -- the rawSpellName is the spell without the rank so we are cool
if not func then return end

func(true)

This issue builds on top of #1294 and #1296