Better support for multiword expressions

[rawlins]

We often want to be able to write something like the following in a %%lamb block (Lucas Champollion):

||a sandwich|| = Lx_e: sandwich(x)
||a_sandwich|| = Lx_e: sandwich(x)

That the second doesn't work is just a bug, underscore is being parsed wrong. The first should probably try to find a usable variable name based on the item name.

Secondary issue: Item.name isn't settable.

[rawlins]

Also, need to add a general solution to the issue of variable name shadowing.

[rawlins]

Quick fix to allow spaces/underscores in item names added in https://github.com/rawlins/lambda-notebook/commit/96ef1c3e66d1784d2a6e8f5f12153c7173d3b33b . This isn't a general solution to the variable name issues though.