Implement limit directive for msgp array/map size bounds checking

[klauspost]

Add //msgp:limit directive to prevent DoS attacks by limiting array/slice and map sizes during msgp serialization/deserialization operations.

Features:

• Unified directive syntax: //msgp:limit arrays:n maps:n marshal:true/false
• Applies only to dynamic slices/maps, excludes fixed arrays (compile-time sized)
• Optional marshal-time enforcement with marshal:true parameter
• Default limits of math.MaxUint32 when not specified
• Returns msgp.ErrLimitExceeded when limits exceeded
• Per-file unique constant generation using CRC32 hash to avoid collisions

Adds msgp:",limit=n" per field tags. This will allow to set or override a per-file limit.

Note: This also applies to []byte slices.

Usage examples:

//msgp:limit arrays:100 maps:50 (unmarshal limits only)

//msgp:limit arrays:100 maps:50 marshal:true (both marshal and unmarshal limits)

type FieldLimitTestData struct {
	SmallSlice []int          `msg:"small_slice,limit=5"`
	LargeSlice []string       `msg:"large_slice,limit=100"`
	SmallMap   map[string]int `msg:"small_map,limit=3"`
	LargeMap   map[int]string `msg:"large_map,limit=20"`
	NoLimit    []byte         `msg:"no_limit"`  // Uses file-level limits if any
	FixedArray [10]int        `msg:"fixed_array,limit=2"` // Should be ignored

Fixes #411