Avoid string allocation of types on statement (and freeze arrays)

[jhawthorn]

We can avoid intermediate allocations by doing the downcasing and interning in C.

This also changes the strings to be UTF-8.

We only downcase ASCII here because that's what was previously done (as the strings used to be binary.

$LOAD_PATH.unshift File.expand_path("../lib", __dir__)
require "sqlite3"
require "ips"

db = SQLite3::Database.new(":memory:")
db.execute "CREATE TABLE t (#{20.times.map { |i| "c#{i} varchar(255)" }.join(", ")})"
sql = "SELECT * FROM t"

IPS.run do |x|
  x.report("prepare + types") do
    stmt = SQLite3::Statement.new(db, sql)
    stmt.types
    stmt.close
  end
end

$ ips --yjit --sha main --sha HEAD bench/metadata.rb
ruby 4.0.7 (2026-09-15 revision 229531a6cf) +YJIT +PRISM [arm64-darwin25]
     prepare + types:   217.697k i/s (± 0.7%, GC  1.8%)

ruby 4.0.7 (2026-09-15 revision 229531a6cf) +YJIT +PRISM [arm64-darwin25]
     prepare + types:   157.717k i/s (± 0.6%, GC  9.4%)


Summary
  faster_types ran
    1.38 ± 0.01 times faster than 576f36c