Added support for asserts on deserialized data

[batconjurer]

Why this should be merged

Closes #1334

Adds assert post-conditions to the #[unpack(...)] macro. Three forms are supported:

• Type-level (assert = "|var| { expr }" on the struct annotation): emits require(expr) just before the function returns, with var substituted by result.
• Field-level (assert = "|var| { expr }" on a field): emits require(expr) immediately after the field is assigned to result, with var substituted by the field's local variable.
• Element-level (assert = "|each var| { expr }" on a field): for array fields, emits require(expr) inside the decode loop per element; for bytes/string/bytesN fields, emits a post-decode byte iteration loop.

Multiple assert keys may appear on a single annotation. The closure value is a quoted string so commas and = signs inside the expression are not misinterpreted as argument separators. each is rejected at the type level and on non-container fields with a descriptive error.

The multi-line comment parser was also fixed: reforge::get_comment concatenates comment lines without newlines, so strip_comment_prefixes now splits on embedded // occurrences (outside quoted strings) in addition to \n.

How this works

How this was tested

How is this documented