Integer literals above i64::MAX crash the compiler #165

Any integer literal larger than i64::MAX panics the compiler, including a valid u64 constant:

const X: u64 = 18446744073709551615

thread 'main' panicked at core/src/schema/idl/grammar.rs:1063:75:
called `Result::unwrap()` on an `Err` value: ParseIntError { kind: PosOverflow }

IntegerLiteral parses -?\d+ into an i64 with s.parse().unwrap(). The same panic happens for a fixed array size (u8[99999999999999999999]) or any other literal.

Fix: parse into a type wide enough for every integer type (i128, or keep the digits as text), and report a literal no type can hold as a diagnostic, not a panic. u64::MAX must stay writable.

Also in the language server and the playground. Both run this parser on what's typed. In the playground (core 5259393, built to wasm), such a literal makes compile, generate (Rust and TypeScript), the simulator's describe_project, hover and completion all fail with RuntimeError: unreachable and a "wasm error" status, until the literal is edited away; only highlighting keeps working. The module recovers afterwards. Typing a u64 constant near its maximum is enough to hit it.

Found on core master (a8aee68).