… value of constants in hover

This change improves the hover information for constants by showing both
the original declaration and the value. The value is displayed as
an inline comment. If the original declaration and the value are the same,
there will be no inline comment.

Examples:

```go
const duration time.Duration = 15*time.Minute + 10*time.Second // 15m10s

const octal untyped int = 0o777 // 511

const expr untyped int = 2 << (0b111&0b101 - 2) // 16

const boolean untyped bool = (55 - 3) == (26 * 2) // true

const dec untyped int = 500
```

Other changes:

* Calls to `objectString` that format methods or variables have been
  replaced with `types.ObjectString`.
* The logic of inferred signature formatting has been extracted from
  `objectString` to a new function `inferredSignatureString`.
* Remove unused function `extractFieldList`.

Fixes golang/go#47453