Add binary map key and shimming

[klauspost]

• 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.

[klauspost]

(considered adding automatic string shimming of built-in types as an option - as the stdlib json encoder does now, but leaving that for later)

[klauspost]

Probably adding autoshim for Uint, Uint8, Uint16, Uint32, Uint64, Int, Int8, Int16, Int32, Int64, Bool, Float32, Float64 as map keys.

Aliases may not be covered yet.

[klauspost]

Added. top post updated.

[klauspost]

@philhofer I will be merging this soon-ish unless you have major objections.