Add basic support for SV file io and poking X values

[leonardt]

Fills out basic support for file IO actions in the system verilog backend, based on the poke-x branch because I needed some SV target related changes

[Kuree]

You can use Ubuntu 16 image for iverilog. It should have a fairly recent version.

dist: xenial

[coveralls]

Pull Request Test Coverage Report for Build 908

• 4 of 69 (5.8%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.3%) to 76.513%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
fault/system_verilog_target.py	4	69	5.8%

Files with Coverage Reduction	New Missed Lines	%
fault/system_verilog_target.py	3	16.57%

Totals
Change from base Build 840:	-1.3%
Covered Lines:	1365
Relevant Lines:	1784

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💛 - Coveralls