Refactor output to be more streaming-focused. (#666)

- Switch code to llvm::raw_ostream as part of standardizing output forms.
    - Preferring llvm::raw_ostream over std::ostream because other tooling code should be expected to rely on llvm more closely, and an overall preference towards library consistency.
    - There are a couple spots in syntax/ that still use std streams, but I'd prefer to take a separate PR to see how best to address those.
    - std::boolalpha doesn't work with llvm, so I've implemented equivalent in a couple places (not enough that it felt like worth making a helper function).
- Implement Print(ostream) as consistently as we can, as an instance member.
    - This facilitates the use of the common/ostream.h template to provide operators.
    - Preferring this approach so that Print is easily accessible via gdb, per suggestion on #executable-semantics.
- Switch code currently calling `type->Print(ostream)` to instead do `ostream << *type`.
- Remove the unused `PrintTypeEnv`, nothing used it and the declaration didn't match the definition.

executable_semantics/ast/BUILD

@@ -17,6 +17,7 @@ cc_library(
         ":function_definition",
         ":member",
         ":struct_definition",
+        "//common:ostream",
         "//executable_semantics/interpreter:address",
         "//executable_semantics/interpreter:containers",
     ],
@@ -26,7 +27,10 @@ cc_library(
     name = "expression",
     srcs = ["expression.cpp"],
     hdrs = ["expression.h"],
-    deps = ["//common:indirect_value"],
+    deps = [
+        "//common:indirect_value",
+        "//common:ostream",
+    ],
 )
 
 cc_library(
@@ -43,7 +47,10 @@ cc_library(
     name = "member",
     srcs = ["member.cpp"],
     hdrs = ["member.h"],
-    deps = [":expression"],
+    deps = [
+        ":expression",
+        "//common:ostream",
+    ],
 )
 
 cc_library(
@@ -53,11 +60,15 @@ cc_library(
     deps = [
         ":expression",
         "//common:check",
+        "//common:ostream",
     ],
 )
 
 cc_library(
     name = "struct_definition",
     hdrs = ["struct_definition.h"],
-    deps = [":member"],
+    deps = [
+        ":member",
+        "//common:ostream",
+    ],
 )