Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

.clang-tidy

@@ -0,0 +1,42 @@
+---
+Checks:          'clang-diagnostic-*,clang-analyzer-*,-*,modernize-use-default-member-init'
+WarningsAsErrors: ''
+HeaderFilterRegex: '.*'
+AnalyzeTemporaryDtors: false
+FormatStyle:     none
+CheckOptions:
+CheckOptions:
+  - key:             cert-dcl16-c.NewSuffixes
+    value:           'L;LL;LU;LLU'
+  - key:             cert-oop54-cpp.WarnOnlyIfThisHasSuspiciousField
+    value:           '0'
+  - key:             cppcoreguidelines-explicit-virtual-functions.IgnoreDestructors
+    value:           '1'
+  - key:             cppcoreguidelines-non-private-member-variables-in-classes.IgnoreClassesWithAllMemberVariablesBeingPublic
+    value:           '1'
+  - key:             google-readability-braces-around-statements.ShortStatementLines
+    value:           '1'
+  - key:             google-readability-function-size.StatementThreshold
+    value:           '800'
+  - key:             google-readability-namespace-comments.ShortNamespaceLines
+    value:           '10'
+  - key:             google-readability-namespace-comments.SpacesBeforeComments
+    value:           '2'
+  - key:             modernize-loop-convert.MaxCopySize
+    value:           '16'
+  - key:             modernize-loop-convert.MinConfidence
+    value:           reasonable
+  - key:             modernize-loop-convert.NamingStyle
+    value:           CamelCase
+  - key:             modernize-pass-by-value.IncludeStyle
+    value:           llvm
+  - key:             modernize-replace-auto-ptr.IncludeStyle
+    value:           llvm
+  - key:             modernize-use-default-member-init.IgnoreMacros
+    value:           '1'
+  - key:             modernize-use-default-member-init.UseAssignment
+    value:           '1'
+  - key:             modernize-use-nullptr.NullMacros
+    value:           'NULL'
+...
+