I keep running into issues due to auto-decoding (arguably a significant design flaw of phobos) when using strings from external sources (which may not be 100% valid UTF8) eg see stracktrace [1] on getSomeExternalString().splitLines,

Could we have something like `sanitizeUTF` in std.utf, to allow for a simple fix when running into such UTF8 issues see proposal implementation [2]; the fix would then be:
```
getSomeExternalString().splitLines,
=>
getSomeExternalString().sanitizeUTF.splitLines,
```

[1]

core.exception.AssertError@std/utf.d(2254): Assertion failure

----------------

??:? _d_assert [0x4f4e63]

??:? void std.utf.__assert(int) [0x53a304]

??:? pure nothrow @nogc @safe ubyte std.utf.codeLength!(char).codeLength(dchar) [0xa5d78191]

??:? pure nothrow @nogc @safe int std.string.stripRight!(immutable(char)[]).stripRight(immutable(char)[]).__foreachbody2(ref ulong, ref dchar) [0xa5c42bd9]

??:? _aApplyRcd2 [0x4f9bd1]

??:? pure @nogc @safe immutable(char)[] std.string.stripRight!(immutable(char)[]).stripRight(immutable(char)[]) [0xa5c42b5c]

??:? pure @property @nogc @safe immutable(char)[] std.algorithm.iteration.stripRight.MapResult.front() [0xa5cda053]

??:? pure @safe immutable(char)[] std.array.join!(std.algorithm.iteration.stripRight.MapResult, immutable(char)[]).join(std.algorithm.iteration.stripRight.MapResult, immutable(char)[]) [0xa5cda39a]

[2] sanitizeUTF proposal:
// TODO: rangeify to make it work in more situations

string sanitizeUTF(string a){

import std.utf;

Appender!string b;

while(a.length){

b~=decodeFront!(Yes.useReplacementDchar)(a);

}

return b.data;

}