Declare many structures const (closes #3552)
Many structs used in mutt are actually constant but are defined without the 'const' keyword. This can slow initialization (slightly) in some environments due to extra copying and increases the amount of writable RAM required at run-time, which can be significant on non-MMU systems. Using const can also increase the opportunities for compiler optimization. The attached patch marks many such structures as const. On my test x86 build, this reduces the size of .data by over 50%.
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