Use C# compiler from Microsoft.Net.Compilers

[Atif Aziz]

Using the Microsoft.Net.Compilers package has the following benefits:

• The C# compiler can be redistributed; currently LINQPadless relies on finding it at known paths where Microsoft Build Tools may be installed
• Enable use of C# 7 with Microsoft.Net.Compilers 2.0.1
• The --csc command-line option can be removed