* Allows for binary map keys
* Allows for non-string map keys to be shimmed as well as cast from string type aliases.
* Allows automatic conversion of some map key types to/from string.

Adds directive `maps`

* `//msgp:maps binkeys|shim|autoshim`

Not adding the directive will retain current behaviour where maps without string keys are ignored.

Adding `binkeys` will allow for binary marshaling of map keys. Types are inferred with standard rules. Binary encoding can be shimmed and can return any standard type. See `_generated/map_bin_key.go` for examples.

Adding `shim` will require you to add shims to custom map key types. Shims must be with string type. See `_generated/map_shim_key.go` for examples.

Adding `autoshim` will add automatic shimming to map keys that are `uint, uint8, uint16, uint32, uint64, int, int8, int16, int32, int64, bool, float32, float64, byte` - so these are written as a string. Standard `strconv.Parse/Format` functions are used. Aliased types are not supported, but `msgp:replace MyInt with:int` directive can be used and this will be picked up.

Adds `witherr:true|false` to `//msgp:shim` directives, so shim de-serializing can return errors.

Updates/Resolves: #331 #345 #305 #257 #232

Quirks:

* Sizing is not perfect and can sometimes be collapsed from a range to constant.
* These maps are not currently readable by "Interface" functions.