feat(builtins): make TextIOWrapper enumerable and readline arg optional

[dbrattli]

Resolves #131.

Problem

builtins.open(...) returns a TextIOWrapper. Two ergonomic gaps were reported:

1. Iterating a file's lines required an untyped cast (!!result) to make for line in result compile.
2. readline forced a size argument (result.readline(-1)), but Python's readline(size=-1) takes it optionally.

Changes

src/stdlib/Builtins.fs:

• Add inherit IEnumerable<string> to TextIOWrapper. Per the bindings guide, IEnumerable<'T> maps to Python's iterator protocol, so for line in reader now compiles directly and types each line as string. A Python file object is its own iterator, so the generated get_enumerator(reader) (→ iter()) works at runtime.
• Add zero-arg readline: unit -> string and readlines: unit -> string list overloads on TextIOBase, matching the existing two-overload pattern already used by read.

The reporter's snippet now works without the cast:

use reader = builtins.``open`` ("test.txt", OpenTextMode.Read)
for (line: string) in reader do
    builtins.print (line.Trim())

Testing

Added two tests to test/TestBuiltins.fs (line enumeration + arg-less readline). Full suite passes — 563 passed.

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