Fix for non-symmetric matrix diagonal fill (Closes #619)

[coatless]

Fix for the non-symmetric case of the diagonal fill.

Adds unit tests to the diagonal fill feature as well.

Per discussion in #619, this PR moves the matrix diagonal fill away from using an iterator solution to using the matrix subset operator.

[codecov-io]

Current coverage is 76.47% (diff: 100%)

Merging #622 into master will not change coverage

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[eddelbuettel]

I was just squinting at this (and mostly wondering if I had written it on one line) when I realized that

 R_xlen_t bounds = std::min( Matrix::nrow(), Matrix::ncol() );

is probably even easier :) (Subjec to testing in whether stdLLmin() likes the R type etc pp)

[eddelbuettel]

I guess one could argue that the ternary operator may be lighter-weight that a function call (which, I bet, ends up inlined). I use that operator a fair amount for alternates too. It just seemed more compact...