DataExpression and StringIdentifier toString readability

Improve readability of StringIdentifier and DataExpression toString methods.

For example, A = matrix("1 2 3 4 5 6", rows=3, cols=2) results in a DMLProgram that contains:

A = RAND(,lambda=1.0,min=1 2 3 4 5 6,pdf=uniform,seed=-1,max=1 2 3 4 5 6,sparsity=1.0,rows=3,cols=2 );

The extraneous first comma within RAND is confusing. Additionally, the string '1 2 3 4 5 6' is not surrounded by quotes, which affects readability.

Readability is improved if the toStrings are modified to produce output such as the following:

A = RAND(lambda=1.0, min='1 2 3 4 5 6', pdf='uniform', seed=-1, max='1 2 3 4 5 6', sparsity=1.0, rows=3, cols=2);