Add Objects and ObjectValues field constructors

[abhinav]

This pull request adds two new field constructors for slices, parameterized
over the types of the values in the slices:

func Objects[T zapcore.ObjectMarshaler](key string, value []T) Field

This turns a slice of any Zap marshalable object into a Zap field.

func ObjectValues[T any, P objectMarshalerPtr[T]](key string, values []T) Field

This is a variant of Objects for the case where *T implements
zapcore.ObjectMarshaler but we have a []T.

Together, these two field constructors obviate the need for writing
ArrayMarshaler implementations for slices of objects:

// Before //////////////

type userArray []*User

func (uu userArray) MarshalLogArray(enc zapcore.ArrayEncoder) error {
    for _, u := range uu {
        if err := enc.AppendObject(u); err != nil {
            return err
        }
    }
    return nil
}

var users []*User = ..
zap.Array("users", userArray(users))

// Now /////////////////

var users []*User = ..
zap.Objects("users", users)

Backwards compatibility

Both new field constructors are hidden behind a build tag, but use of type
parameters requires us to bump the go directive in the go.mod file to 1.18.

Note that this does not break Zap on Go 1.17.
Per the documentation for the go directive,

If an older Go version builds one of the module’s packages and encounters a
compile error, the error notes that the module was written for a newer Go
version.

It only breaks our ability to run go mod tidy inside Zap from Go 1.17,
which is okay because with the go directive, we're stating that we're
developing Zap while on Go 1.18.

Linting

staticcheck support for type parameters is expected in a few weeks.
Meanwhile, this disables staticcheck for the make lint target.
(I considered switching linting to Go 1.17 until then, but that isn't
enough because the formatting linter expects valid Go code--which type
parameters aren't in 1.17.)

[codecov]

Codecov Report

Merging #1071 (bff2745) into master (34cb012) will increase coverage by 0.10%.
The diff coverage is 100.00%.

❗ Current head bff2745 differs from pull request most recent head 854d895. Consider uploading reports for the commit 854d895 to get more accurate results

@@            Coverage Diff             @@
##           master    #1071      +/-   ##
==========================================
+ Coverage   98.19%   98.30%   +0.10%     
==========================================
  Files          48       49       +1     
  Lines        2111     2120       +9     
==========================================
+ Hits         2073     2084      +11     
+ Misses         29       28       -1     
+ Partials        9        8       -1     

Impacted Files	Coverage Δ
array_go118.go	100.00% <100.00%> (ø)
zapcore/sampler.go	100.00% <0.00%> (+3.77%)	⬆️

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[sywhang]

neat :) left a minor nit on the comment but LGTM otherwise.