go/format, cmd/gofmt: fix issues with partial Go code with indent

Fixes issue 5551.
Fixes issue 4449.

Adds tests for both issues.

Note that the two issues occur only when formatting partial Go code
with indent.

The best way to understand the change is as follows: I took the code
of cmd/gofmt and go/format, combined it into one unified code that
does not suffer from either 4449 nor 5551, and then applied that code
to both cmd/gofmt and go/format.

As a result, there is now much more identical code between the two
packages, making future code deduplication easier (it was not possible
to do that now without adding public APIs, which I was advised not to
do at this time).

More specifically, I took the parse() of cmd/gofmt which correctly
preserves comments (issue 5551) and modified it to fix issue where
it would sometimes modify literal values (issue 4449).

I ended up removing the matchSpace() function because it no longer
needed to do some of its work (insert indent), and a part of its work
had to be done in advance (determining the indentation of first code
line), because that calculation is required for cfg.Fprint() to run.

adjustIndent is used to adjust the indent of cfg.Fprint() to compensate
for the body of wrapper func being indented by one level. This allows
to get rid of the bytes.Replace text manipulation of inner content,
which was problematic and sometimes altered raw string literals (issue
4449). This means that sometimes the value of cfg.Indent is negative,
but that works as expected.

So now the algorithm for formatting partial Go code is:

1. Determine and prepend leading space of original source.
2. Determine and prepend indentation of first code line.
3. Format and write partial Go code (with all of its leading &
   trailing space trimmed).
4. Determine and append trailing space of original source.

LGTM=gri
R=golang-codereviews, bradfitz, gri
CC=golang-codereviews
https://codereview.appspot.com/142360043

Committer: Robert Griesemer <gri@golang.org>