[Refactor] Mark whole `std.array.Appender` functions `@trusted`.

`Appender` do a low-level job so it's already hard to implement properly and contains some errors. Needless code complication caused by nested `@trusted` functions is unacceptable.

Partially revert 9cdfd6899af02ffde91ac2cdc30170db5396f2db and small part of 99d22b80f657fae485d837872eeea3e142ae32d4.

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The only "not a good" thing is this pull I can see is that it makes `put` `@trusted` in some cases when it can be `@safe` but I see nothing bad in it.