Reduce memory usage of RBS::Buffer

[pocke]

This PR reduces memory usage of RBS::Buffer.

RBS::Buffer setups @lines and @ranges on initialize, but they aren't used in many cases and they consume large memory.
This patch makes the initialization lazy to avoid allocating memory.

It doesn't affect rbs command speed, but it reduces memory usage.

Benchmarking

It reduces 8,275kb (17%) memory only with the core libraries, and reduces 33,718kb (24%) memory with rails gems.

With core libraries

$LOAD_PATH << File.join(__dir__, "./lib")
require 'rbs'
require 'rbs/cli'

def rss
  `ps u --no-headers --pid #{Process.pid}`.split(' ')[5].to_i
end

l = RBS::EnvironmentLoader.new
x = x = RBS::Environment.from_loader(l).resolve_type_names
GC.start
p rss

before: 48690 (kb)
after: 40415 (kb)

With rails gems

$LOAD_PATH << File.join(__dir__, "./lib")
require 'rbs'
require 'rbs/cli'

def rss
  `ps u --no-headers --pid #{Process.pid}`.split(' ')[5].to_i
end

l = RBS::EnvironmentLoader.new
path = Pathname('/path/to/rbs_collection.yaml')
lock = RBS::Collection::Config.lockfile_of(path)
l.add_collection lock
x = x = RBS::Environment.from_loader(l).resolve_type_names
GC.start
p rss

before: 139662 (kb)
after: 105944 (kb)

[soutaro]

👍

(The methods are used by Location if the instance is not created during parsing.)