Allow implicit conversion to C types by defining a `to_unsafe` method. This removed the hardcoded rule for converting a `String` to `UInt8*` and also allows passing an `Array(T)` to an argument expecting `Pointer(T)`.

Added expect_raises macros and use them in specs. Also make assert_syntax_error create a spec with "it" (more concise)

Lexer: check that integer literals fit their type. Also: deduce their type to Int32, Int64 or UInt64 depending on their value.

Replaced `@name` with `@class_name` inside macros. And: instance vars inside macros don't have the `@` in their name anymore.

Make string interpolation create a StringIO with some guessed capacity to avoid memory reallocations if possible

Don't to_s Call node as "foo()" if it's just "foo" without arguments (more useful for macros, for pasting code)

Much faster Int#to_s without using sprintf. Also: removed append_c_string from StringBuffer and make sure to put an ending null character in String#new_with_capacity_and_length

New convention for to_s and inspect: you must define them receiving an IO-like object to which to append. This prevents useless memory allocations by avoiding allocating unnecessary intermediate strings.

String.build now uses StringBuffer. Added some specs for StringBuffer and changed `append_byte` to `<<(byte : UInt8)`.