Restrict all context.{get,set} in same component to use same elem type

[lukewagner]

This PR adds validation rules that require all context.get and context.set definitions inside a component to use the same element type (currently either i32 or, with #624, i64). Conceptually, this validation rule would be attached to what it means for a component definition to be valid. In the short-term, this might allow context.set i32 to avoid a store-zero when implemented on 32-bit CPUs (to clear the high bits of the underlying i64 storage) if there's no whole-component information to indicate the absence of context.set i64s in the same component. In the longer-term, this change would allow core wasm reference types to be efficiently used as element types, since the simple aliasing approach of i32/i64 wouldn't work.

(cc @adambratschikaye)

[lukewagner]

Additionally, the second commit tweaks the binary format rules in Binary.md to use validation to gate context.{get,set} and (type (resource (rep v))) to i32/i64 instead of decoding rules. This mostly only shows up in whether WAST tests report a decoding vs. validation error, as highlighted in wasm-tools/#2502.