fgm/waiig15: My version of the Writing an Interpreter in Go 1.5 code. @ 3f83d07e0ae174171ad6b69966250fe6d60870e5

Frederic G. MARAND 3f83d07e0a §2.5 parsing return statements. One pass of golint cleanup.		6 lat temu
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ast.go	3f83d07e0a §2.5 parsing return statements. One pass of golint cleanup.	6 lat temu
identifier.go	3f83d07e0a §2.5 parsing return statements. One pass of golint cleanup.	6 lat temu
let.go	3f83d07e0a §2.5 parsing return statements. One pass of golint cleanup.	6 lat temu
return.go	3f83d07e0a §2.5 parsing return statements. One pass of golint cleanup.	6 lat temu