doc: first draft of Go 1.7 release notes

Mostly complete but a few TODOs remain for future CLs.

For #15810.

Change-Id: I81ee19d1088d192cf709a5f7e6b7bcc44ad892ac
Reviewed-on: https://go-review.googlesource.com/23379Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

doc/go1.7.txt

-Tools:
-
-cmd/dist: add list subcommand to list all supported platforms (CL 19837)
-cmd/go: GO15VENDOREXPERIMENT gone, assumed on (CL 19615)
-cmd/link: "-X name value" form gone (CL 19614)
-cmd/compile: smaller binaries (many CLs)
-cmd/go, go/build: add support for Fortran (CL 19670, CL 4114)
-cmd/doc: group constructors with types (CL 22354)
-cmd/go, go/build: binary-only package support (CL 22433)
-cmd/trace: file:line is embed into traces (CL 21732) which makes binary argument optional (CL 22410)
-cmd/trace: now supports visualization of very large traces by splitting them into parts (CL 22731)
-cmd/trace: tracing has become significantly faster (CL 21512)
-
-Ports:
-
-We now require OpenBSD 5.6+ (CL 18219, crypto/rand using getentropy)
-plan9/arm support? Start at least.
-cgo and external linking support for linux/mips64 and linux/mips64le (CL 19809, ...)
-
-New packages:
-
-* context (and new support in net, net/http, os/exec, net/http/httptrace)
-* net/http/httptrace
-
-API additions and behavior changes:
-
-crypto/tls: allow renegotiation to be handled by a client (CL 22475)
-runtime: support symbolic backtrace of C code in a cgo crash (CL 17761)
-runtime: add CallerFrames and Frames (CL 19869)
-runtime: add KeepAlive (CL 23102)
-testing/quick: now generates nil values (CL 16470)
-net/http/httptest: ResponseRecorder supports trailer (CL 20047) (compat impact: issue 14928)
-net/url: support query string without values (CL 19931)
-net/textproto: permit all valid token chars in CanonicalMIMEHeaderKey input (CL 18725)
-go/doc: add Unordered boolean to Example struct (CL 19280)
-time: print zero duration as 0s, not 0 (CL 22357)