[levno-710]

Fixes five Lua 5.1 compatibility issues found through adversarial testing:

• Normalize long-string line endings and correctly omit the initial newline.
• Evaluate surplus assignment expressions and local initializers for side effects and errors, while still skipping literals.
• Avoid unintended function environment lookups when Cff returns.
• Preserve negative zero in Vmify and Cff, including when positive and negative zero appear together.
• Preserve native Lua 5.1 write order in multiple assignments.

Adds regression tests for each issue, including nested functions, strict environments, metatable side effects, and spilled registers.