own: a value pushed where not null; literal unions as views; moves on every path; constant literal globals (TSLANG 67-70); string literal union type check - #855

Fixes from the -mm=own sweep of native_ts_graphics (TSLANG 67-70, written up in TSLANG_ISSUES.md), plus a type check for unions of string literals.

-mm=own

• 67: a value pushed only where it is not null (if (x) arr.push(x); use(x)) is read back from the array on the pushed path and is null on the other.
• 68: a union of string literals, like a string, is read as a copyable view.
• 69: a value moved on every path, by a different move on each, is no longer refused as moved on some paths only.
• 70: a constant global of a string literal's type, given to a call that keeps it, is an immortal literal.

Type checking

• A union of string literals ("no" | "yes") takes only its literals. Another literal, or a string, given to it is an error (TS2322 for assignments and declarations, TS2345 for arguments).
• A parameter property typed by its default (private flags = "") has the default's widened type (string), as in TS. DefaultLib's RegExp depends on this.

Tests

• Every model: 00tslang_issue_67, 00tslang_issue_69.
• Error tests:
  • assign_string_literal_union_error
  • assign_string_to_literal_union_error
  • argument_string_literal_union_error
  • own_err_pushed_or_null_popped
  • own_err_moved_on_two_of_three_paths
• The literal-union error tests are by design and excluded from the ownership verifier.

Validation

• ctest: 7331 of 7331 passed.
• DefaultLib tests.ps1: all four models passed. Results are in a comment.
• ntg sweep: 639 errors before these changes, 635 after.

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