[refactor] Mark all internal functions static

All packet handlers and helper functions that are only used in the
source file they're declared in have been taken out of the namespaces in
the corresponding header files, have been marked static, and have been
reordered to avoid the need for declarations at the top of each source
file.

Each source file now contains a "using namespace" directive so that the
static functions don't need to prefix the source file's symbols with
their namespace. All redundant namespace prefixes found have been
removed.

An unused nano power resetting function in NanoManager has been removed.

src/Abilities.hpp

@@ -55,6 +55,8 @@ namespace NanoManager {
 
     void nanoUnbuff(CNSocket* sock, std::vector<int> targetData, int32_t bitFlag, int16_t timeBuffID, int16_t amount, bool groupPower);
     int applyBuff(CNSocket* sock, int skillID, int eTBU, int eTBT, int32_t groupFlags);
+
+    std::vector<int> findTargets(Player* plr, int skillID, CNPacketData* data = nullptr);
 }
 
 namespace Combat {