rename: space to table in Lua API

Summary

Rename all appearances of space to table.

In Lua API:

• pico.create_table
• pico.drop_table
• pico.grant_privilege('user', 'read', 'table')
• pico.revoke_privilege('user', 'read', 'table')
• pico.cas({table = ...}, {ranges = {{ table = ... }}})
• pico.help('table CasRange')
• pico.help('table TableField')

In Rust:

• op::Dml::* { table: SpaceId }, op::DmlInLua { table: String }
• cas::Range { table: SpaceId }, cas::RangeInLua {table: Stirng }

Part of #358 (closed)

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if API docs changed) A follow-up doc issue is created: docs#89 (closed)