support custom normalization data

Support custom normalization data, especially for the mappings/decompositions. This is useful for UTS !#46 (for IDNAbis), combining NFKC with case folding (new NFKC_Casefold property in Unicode 5.2) and other variations of "Unicode normalization except...".

Related to

• #5072 add function for normalization + case folding

• #7144 Support TR46 (IDNA Update)

• #7202 Update StringPrep