Bring back `namespace` blocks and allow them to contain type decls, term defs, etc

[pchiusano]

Namespace blocks have some weird limitations:

• Can't have type declarations in them
• Doesn't parse to have use at the top of a namespace block
• I think if you say namespace Optional where ..., you still have to use Optional None Some.

Related: #380

[pchiusano]

We determined that by adding some extra stuff to ParsingEnv, we could make it so namespace blocks can have type decls, term decls, use statements...

[pchiusano]

Also, if it's easy: support namespace .bar where ..., allow absolute names for namespaces, so easy to declare definitions in paths that aren't under current namespace.

[aryairani]

Also related #1299

Also @Stew mentioned they should contain project names too, which made sense until I started to write it down. Will need to ask him for clarification. What do you do with multiple blocks for different projects in the same scratch file on save?