[5.7] Support index length on MySQL

[staudenmeir]

This PR allows MySQL migrations to specify the index length:

Schema::create('table', function(Blueprint $table) {
    $table->string('a');
    $table->string('b');
    $table->text('c');
    $table->primary(['a', 'b'], null, null, [10, 20]);
    $table->index('c', null, null, 50);
});

This is especially useful for TEXT columns, as they don't allow indexes without a length.

PostgreSQL, SQLite and SQL Server don't support index lengths. There is a workaround for PostgreSQL, but it only supports non-unique indexes and is cumbersome to use in queries. So I don't think it's worth the effort.

I didn't add exceptions for the other databases because I don't think they are necessary for a such a minor feature.

Resolves #9293.

[sisve]

PostgreSQL, SQLite and SQL Server don't support index lengths.

I think SQLite supports it via expression indexes, but I havn't looked into it.

I didn't add exceptions for the other databases because I don't think they are necessary for a such a minor feature.

I disagree with this part.

[mfn]

I didn't add exceptions for the other databases because I don't think they are necessary for a such a minor feature.

This is one of the major pain points with the current migrations and the Fluent approach: unsupported stuff totally goes unnnoticed until you realize later it didn't work.

Great example from practice: you switch from MySQL to Postgres.

You may say "edge case", I say unnecessary pain point because of "laziness" (note: this isn't meant a diss of your work 👍 , just how it might turn out in the end).

[taylorotwell]

No plans to add this at this time.

[driesvints]

@staudenmeir @sisve @mfn we're reconsidering this for Laravel 7. Would any of you be up to create a PR for the master branch? I think support for other DB engines is wanted as well.