Mandatory `+spdx`

Special meta +spdx should be present in each .eo program.

Incorrect:

# Foo.
[] > foo

Correct:

+spdx SPDX-License-Identifier: MIT

# Foo.
[] > foo

In XMIR, spdx meta should look like this:

<meta>
  <head>spdx</head>
  <tail>SPDX-License-Identifier: MIT</tail>
  <part>SPDX-License-Identifier:</part>
  <part>MIT</part>
</meta>

Mandatory +spdx