Implement issue# 10314.

This moves std.traits.unsigned to std.conv and adds std.conv.signed,
since for some reason, we have std.traits.unsigned but do not have
std.traits.signed.
Fix trivial nitpicks
[Refactoring] Delaying throwing Exception makes control flow analysis simple
[Refactoring] Define enforceFmt
fix Issue 10254 - Purity correctness is broken with constructor
[appender] Reuse ensureAddable for reserve implementation

AFAICS, they have mostly same code.
[appender] Accepting qualified buffer is not safe behavior

To reduce existing code breaking, mark such construction as @system.
[appender] Determine @trusted operations
[parse, str.to!x] Determine @trusted operations
[formatValue] (bool, null, integrals, floatings, chars)

Floating point is blocked by sprintf
[non to!string] because they are unrelated to formatValue
[format] Add @safe/pure ability to writeUpToNextSpec, fillUp, and formattedWrite
[formatValue] (pointer)

+ Infer attributes in retro range primitives
[to!string] severals
[appender] Mark pointer slicing @trusted (fix for issue 10359)
Ensure adequate code coverage from std.random.RandomSample unittests.

The old unittests were missing coverage of large sections of
RandomSample.skip() because the ratios of sample size to source size
were such that Algorithm A was always being chosen over Algorithm D.

The new unittests fix this and also extend coverage to a number of
other areas previously not addressed.  There are also currently-failing
tests commented out with version(none) that can be used to verify
future bugfixes and improvements.

A few lines in skip() remain uncovered simply because they depend on
runtime conditions that are extremely rare.
Complete code coverage for Xorshift unittests.

Unittests for Xorshift were previously failing to cover the
constructor, and the .seed(), .save and .opEquals() methods.
Complete non-static code coverage for LinearCongruentialEngine unittests.

Unittests for LinearCongruentialEngine were previously missing the
constructor and the .save and .opEquals() methods.

Code coverage analysis still fails to pick up on the static methods
and the contents of a static if scope in lines 374-382 of .popFront().
Unittest for MersenneTwisterEngine .save.
Remove superflous assert(false) and return statements.