Sort operations to guarantee a stable order (#156)

Problem: currently when using a wildcard that covers multiple files in `operations:` YAML directive the generator emits functions in a non-stable ordering due to the use of map for storing file names:

https://github.com/Khan/genqlient/blob/e0accbded177db9143314911bacedf61b2cda656/generate/parse.go#L68-L84

Solution: sort the operations before emitting them.

go.sum

@@ -14,7 +14,6 @@ github.com/arbovm/levenshtein v0.0.0-20160628152529-48b4e1c0c4d0 h1:jfIu9sQUG6Ig
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@@ -36,7 +35,6 @@ github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
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@@ -45,17 +43,14 @@ github.com/mitchellh/mapstructure v0.0.0-20180203102830-a4e142e9c047 h1:zCoDWFD5
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@@ -66,9 +61,7 @@ github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81P
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