0.7.0 - Multi-Symbol Imports

Adds support for importing multiple elements/symbols in one import statement:

import emerge.std.io.{IOException, PrintStream}

It also adds a couple other improvements around imports:

• if the same element/symbol is imported multiple times, you get an INFO diagnostic
• if multiple elements/symbols from different packages with identical simple name are imported, you get an ERROR diagnostic
• if a type reference is ambiguous due to wildcard imports, you get an ERROR diagnostic (before it would just silently resolve to the first type imported)
• if you declare two global variables with identical names in the same package (and they both have visibility beyond their file), then you get an ERROR diagnostic
• the ERROR diagnostic around circular variable initialization is now much clearer

Also, some refactorings:

• BoundBaseType.baseReference got replaced with getBoundReferenceAssertNoTypeParameters(Span) and getBoundReferenceWithNoneOrWildcardTypeArguments(Span). This makes a footgun more obvious and also makes sure these type references have a proper Span wherever possible
• type inference in BoundVariable was refactored; the data and logic flow is now much more obvious from the code