feat: implement mapping evaluator with built-in functions (#45)

- Implement statement evaluator (eval.rs) supporting all 6 statement
  types: rename, select, drop, set, default, cast
- Path resolution with nested field access, array indices, and wildcards
- Expression evaluation with operator precedence: arithmetic (+, -, *, /,
  %), comparison (==, !=, >, >=, <, <=), logical (and, or, not)
- String concatenation via + operator with auto-coercion
- Type casting: int, float, string, bool with null handling
- Division/modulo by zero error detection
- Mixed int/float arithmetic with automatic promotion
- Truthiness semantics for all value types
- Implement 30+ built-in functions (functions.rs):
  String: lower, upper, trim, trim_start, trim_end, len, replace,
  contains, starts_with, ends_with, substr, concat, split, join, reverse
  Type: to_int, to_float, to_string, to_bool, type_of
  Math: abs, min, max, floor, ceil, round
  Null: is_null, coalesce, default
- Function aliases: lowercase/downcase, uppercase/upcase, length/size,
  ltrim/rtrim, substring, int/float/str/string/bool/typeof
- Add 110+ tests covering: all statement types with various inputs,
  path resolution (nested, array, wildcard), expression evaluation,
  function calls (direct and nested), type casting, error cases,
  multi-statement pipelines, complex programs, edge cases

Fixes #17

Author: alvinreal