Use Std.isOfType instead of Std.is

[jstolarek]

Std.is is now deprecated and causes warnings with latest Haxe 4.2:

~/.haxelib/hexlog/1,0,0-alpha,7/src/hex/log/filter/AbstractFilterable.hx:28: characters 12-48 : Warning : Std.is is deprecated. Use Std.isOfType instead.
~/.haxelib/hexlog/1,0,0-alpha,7/src/hex/log/filter/AbstractFilterable.hx:48: characters 12-48 : Warning : Std.is is deprecated. Use Std.isOfType instead.
~/.haxelib/hexlog/1,0,0-alpha,7/src/hex/log/internal/AbstractLogger.hx:88: characters 7-30 : Warning : Std.is is deprecated. Use Std.isOfType instead.
~/.haxelib/hexlog/1,0,0-alpha,7/src/hex/log/filter/CompositeFilter.hx:29: characters 7-38 : Warning : Std.is is deprecated. Use Std.isOfType instead.
~/.haxelib/hexlog/1,0,0-alpha,7/src/hex/log/filter/CompositeFilter.hx:47: characters 7-38 : Warning : Std.is is deprecated. Use Std.isOfType instead.

This simple PR silences the warnings.