Implement first pass at MySQL JOIN support (#690)

* endtoend: Add MySQL identical tables test

* endtoend: Add MySQL join where clause test

* dolphin: Implement first pass at JOIN support

* dolphin: Convert ast.JoinExpr

Author: kyleconroy

internal/endtoend/testdata/identical_tables/go/db.go renamed to internal/endtoend/testdata/identical_tables/mysql/go/db.go

@@ -3,6 +3,7 @@
   "packages": [
     {
       "path": "go",
+      "engine": "mysql:beta",
       "name": "querytest",
       "schema": "query.sql",
       "queries": "query.sql"

internal/endtoend/testdata/identical_tables/go/models.go renamed to internal/endtoend/testdata/identical_tables/mysql/go/models.go

@@ -0,0 +1,5 @@
+CREATE TABLE foo (id text not null);
+CREATE TABLE bar (id text not null);
+
+-- name: IdenticalTable :many
+SELECT * FROM foo;

internal/endtoend/testdata/identical_tables/go/query.sql.go renamed to internal/endtoend/testdata/identical_tables/mysql/go/query.sql.go

@@ -3,6 +3,7 @@
   "packages": [
     {
       "path": "go",
+      "engine": "postgresql",
       "name": "querytest",
       "schema": "query.sql",
       "queries": "query.sql"