Only use function barrier for VariableTimeZone

Adding the function barrier with FixedTimeZone ended up increasing
allocations for this benchmark:

```julia
@benchmark collect($(ZonedDateTime(2020, tz"America/Winnipeg"):Hour(1):ZonedDateTime(2021, tz"America/Winnipeg")))
```

Author: omus

src/types/zoneddatetime.jl

@@ -32,29 +32,26 @@ keyword is true the given `DateTime` is assumed to be in UTC instead of in local
 converted to the specified `TimeZone`.  Note that when `from_utc` is true the given
 `DateTime` will always exists and is never ambiguous.
 """
-function ZonedDateTime(dt::DateTime, tz::TimeZone; from_utc::Bool=false)
-    _ZonedDateTime(dt, tz, from_utc ? UTC : Local)
-end
-
-function _ZonedDateTime(dt::DateTime, tz::VariableTimeZone, T::Type{<:Union{Local,UTC}})
-    possible = interpret(dt, tz, T)
-
-    num = length(possible)
-    if num == 1
-        return first(possible)
-    elseif num == 0
-        throw(NonExistentTimeError(dt, tz))
-    else
-        throw(AmbiguousTimeError(dt, tz))
+function ZonedDateTime(dt::DateTime, tz::VariableTimeZone; from_utc::Bool=false)
+    # Note: Using a function barrier which reduces allocations
+    function construct(T::Type{<:Union{Local,UTC}})
+        possible = interpret(dt, tz, T)
+
+        num = length(possible)
+        if num == 1
+            return first(possible)
+        elseif num == 0
+            throw(NonExistentTimeError(dt, tz))
+        else
+            throw(AmbiguousTimeError(dt, tz))
+        end
     end
-end
 
-function _ZonedDateTime(local_dt::DateTime, tz::FixedTimeZone, ::Type{Local})
-    utc_dt = local_dt - tz.offset
-    return ZonedDateTime(utc_dt, tz, tz)
+    return construct(from_utc ? UTC : Local)
 end
 
-function _ZonedDateTime(utc_dt::DateTime, tz::FixedTimeZone, ::Type{UTC})
+function ZonedDateTime(dt::DateTime, tz::FixedTimeZone; from_utc::Bool=false)
+    utc_dt = from_utc ? dt : dt - tz.offset
     return ZonedDateTime(utc_dt, tz, tz)
 end