Fix/codegen operation

[barbieri]

Fix issues with the code generator.

One issue was introduced by #123, the other is the handling of aliases which was mutating the frozen list.

Operations codegen now also generates simpler code by using top level variables _schema and _schema_root.

Fix __to_graphql_input__ when variables are used for list types, this was reported in #125 as well.

Setup CI so more tests are done, avoiding bugs such as introduced by PR #123.

Fixes: #125

[coveralls]

Pull Request Test Coverage Report for Build 267

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 174:	0.0%
Covered Lines:	1468
Relevant Lines:	1468

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💛 - Coveralls