Immutable versions of String Trim methods
Eliminate SStr_Assign in TestIndexFileNames
Eliminate SStr_Assign_Str in Index::DocReader
Eliminate SStr_Assign_Trusted_Str in XSBind.c
Eliminate SStr_Assign_Str in boot.c
Rework Doc_Extract

Remove the ViewCharBuf argument and return an incremented object.
Eliminate ViewCB_Assign in Index::Inverter
Remove Assign methods from ViewCharBuf
Make SortCache_Value work with immutable strings
Eliminate Str_Mimic_Str in PostingPool
Convert TextTermStepper to CharBuf
Eliminate Obj_Mimic in SortFieldWriter

This is probably wrong but it doesn't break any tests.
Remove Grow, Mimic, and Set_Size methods from String
Remove 'cap' field from String
Remove unused S_die_invalid_pattern from String.c
Remove Str_new and Str_init
Optimize Str_compare

UTF-8 strings can be (naively) compared using memcmp.